Add tests for the example KDL files (#1)

* Add From impls for KdlNodeValue

Most clients probably don't need to create these directly, but we may as
well make it easy for them.

* Add TryFrom impls to convert from KdlNodeValue to underlying types

This includes converting from borrowed values, which will produce `&str`
instead of `String`.

* Add test for parsing examples/ci.kdl

This tests that the file can parse, and that its contents match what we
expect. This test currently fails as the file cannot be parsed.

I may have gone a little overboard in the macro I wrote in order to
generate the expected nodes.

* Add test for parsing examples/cargo.kdl

* Add test for examples/nuget.kdl

Unlike the others, this one just validates that it parses, as the file
is sufficiently large that I don't want to copy its contents into the
test.

tests/examples.rs

@@ -0,0 +1,136 @@
+//! Tests the kdl files in the examples directory.
+
+use kdl::*;
+use std::collections::HashMap;
+
+/// Helper for constructing nodes.
+///
+/// This takes input that's similar to KDL itself, but each node must be terminated with either
+/// a semicolon or a braced block. Nodes whose name contains characters not valid in Rust
+/// identifiers must be written as a string literal instead.
+macro_rules! nodes {
+    ([$v:ident]:name) => {};
+    ([$v:ident]:name $name:ident $($tt:tt)*) => {
+        nodes!([$v]:values stringify!($name); {} {} $($tt)*)
+    };
+    ([$v:ident]:name $name:literal $($tt:tt)*) => {
+        nodes!([$v]:values $name; {} {} $($tt)*)
+    };
+    ([$v:ident]:values $name:expr; {$($value:literal,)*} $props:tt $new_value:literal $($tt:tt)*) => {
+        nodes!([$v]:values $name; {$($value,)* $new_value,} $props $($tt)*)
+    };
+    ([$v:ident]:values $name:expr; $values:tt {$($key:ident=$prop:literal,)*} $new_key:ident=$new_prop:literal $($tt:tt)*) => {
+        nodes!([$v]:values $name; $values {$($key=$prop,)* $new_key=$new_prop,} $($tt)*)
+    };
+    ([$v:ident]:values $name:expr; $values:tt $props:tt $(; $($tt:tt)*)?) => {
+        nodes!([$v]:values $name; $values $props {} $($($tt)*)?)
+    };
+    ([$v:ident]:values $name:expr; {$($value:literal,)*} {$($key:ident=$prop:literal,)*} {$($child:tt)*} $($tail:tt)*) => {
+        $v.push(KdlNode {
+            name: $name.to_owned(),
+            values: vec![$( $value.to_owned().into() ),*],
+            properties: {
+                #[allow(unused_mut)]
+                let mut map = HashMap::new();
+                $(
+                    map.insert(stringify!($key).to_owned(), $prop.to_owned().into());
+                )*
+                map
+            },
+            children: nodes!($($child)*),
+        });
+        nodes!([$v]:name $($tail)*);
+    };
+    // Explicitly match literal and ident at the start instead of $($tt:tt)*
+    // so we get better errors than "recursion limit exceeded" if we fail to match.
+    (:start $($tt:tt)+) => {{
+        let mut v = Vec::new();
+        nodes!([v]:name $($tt)+);
+        v
+    }};
+    ($name:literal $($tt:tt)*) => {
+        nodes!(:start $name $($tt)*)
+    };
+    ($name:ident $($tt:tt)*) => {
+        nodes!(:start $name $($tt)*)
+    };
+    () => { vec![] }
+}
+
+#[test]
+fn test_ci() {
+    let doc = parse_document(include_str!("../examples/ci.kdl"));
+    let nodes = nodes! {
+        name "CI";
+        on "push" "pull_request";
+        env {
+            RUSTFLAGS "-Dwarnings"
+        }
+        jobs {
+            fmt_and_docs "Check fmt & build docs" {
+                "runs-on" "ubuntu-latest";
+                steps {
+                    step uses="actions/checkout@v1";
+                    step "Install Rust" uses="actions-rs/toolchain@v1" {
+                        profile "minimal";
+                        toolchain "stable";
+                        components "rustfmt";
+                        override true;
+                    }
+                    step "rustfmt" run="cargo fmt --all -- --check";
+                    step "docs" run="cargo doc --no-deps";
+                }
+            }
+            build_and_test "Build & Test" {
+                "runs-on" "${{ matrix.os }}";
+                strategy {
+                    matrix {
+                        rust "1.46.0" "stable";
+                        os "ubuntu-latest" "macOS-latest" "windows-latest";
+                    }
+                }
+
+                steps {
+                    step uses="actions/checkout@v1";
+                    step "Install Rust" uses="actions-rs/toolchain@v1" {
+                        profile "minimal";
+                        toolchain "${{ matrix.rust }}";
+                        components "clippy";
+                        override true;
+                    }
+                    step "Clippy" run="cargo clippy --all -- -D warnings";
+                    step "Run tests" run="cargo test --all --verbose";
+                }
+            }
+        }
+    };
+    assert_eq!(doc, Ok(nodes));
+}
+
+#[test]
+fn test_cargo() {
+    let doc = parse_document(include_str!("../examples/cargo.kdl"));
+    let nodes = nodes! {
+        package {
+            name "kdl";
+            version "0.0.0";
+            description "kat's document language";
+            authors "Kat Marchán <kzm@zkat.tech>";
+            "license-file" "LICENSE.md";
+            edition "2018";
+        }
+        dependencies {
+            nom "6.0.1";
+            thiserror "1.0.22";
+        }
+    };
+    assert_eq!(doc, Ok(nodes));
+}
+
+#[test]
+fn test_nuget() {
+    let doc = parse_document(include_str!("../examples/nuget.kdl"));
+    // This file is particularly large. It would be nice to validate it, but for now
+    // I'm just going to settle for making sure it parses.
+    doc.expect("Parsing failed");
+}