feat(len): add APIs to calculate component lengths (#36)

Fixes: https://github.com/kdl-org/kdl-rs/issues/19

src/document.rs

@@ -168,6 +168,16 @@ impl KdlDocument {
         self.trailing = Some(trailing.into());
     }
 
+    /// Length of this document when rendered as a string.
+    pub fn len(&self) -> usize {
+        format!("{}", self).len()
+    }
+
+    /// Returns true if this document is completely empty (including whitespace)
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Clears leading and trailing text (whitespace, comments). `KdlNode`s in
     /// this document will be unaffected.
     pub fn clear_fmt(&mut self) {

src/entry.rs

@@ -98,6 +98,16 @@ impl KdlEntry {
         self.value_repr = Some(repr.into());
     }
 
+    /// Length of this entry when rendered as a string.
+    pub fn len(&self) -> usize {
+        format!("{}", self).len()
+    }
+
+    /// Returns true if this entry is completely empty (including whitespace).
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Auto-formats this entry.
     pub fn fmt(&mut self) {
         self.leading = None;

src/identifier.rs

@@ -31,6 +31,16 @@ impl KdlIdentifier {
         self.repr = Some(repr.into());
     }
 
+    /// Length of this identifier when rendered as a string.
+    pub fn len(&self) -> usize {
+        format!("{}", self).len()
+    }
+
+    /// Returns true if this identifier is completely empty.
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Resets this identifier to its default representation. It will attempt
     /// to make it an unquoted identifier, and fall back to a string
     /// representation if that would be invalid.

src/node.rs

@@ -106,6 +106,16 @@ impl KdlNode {
         self.trailing = Some(trailing.into());
     }
 
+    /// Length of this node when rendered as a string.
+    pub fn len(&self) -> usize {
+        format!("{}", self).len()
+    }
+
+    /// Returns true if this node is completely empty (including whitespace).
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Clears leading and trailing text (whitespace, comments), as well as
     /// the space before the children block, if any. Individual entries and
     /// their formatting will be preserved.