* 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.
Lily Ballard