Add YamlLoader::load_from_str_with_markers to provide AST with source markers

[hallettj]

fixes #103, replaces #105

The new function produces an alternative representation for YAML
documents where each YAML node is paired with a Marker to indicate the
corresponding line and column in the source markup. The new
representation takes the form of two new types, Node and YamlMarked.

Node is a pair of YamlMarked and Marker.

YamlMarked mimics the existing Yaml enum; the difference is that array
elements and hash keys and values are Node values instead of Yaml or
YamlMarked values. I created a new enum because I did not know of a way
to switch child nodes in Yaml between Yaml and Node types without
backward-incompatible changes to the Yaml enum.

The the behavior of the existing load_from_str function and Yaml
enum are unchanged, so pattern matching on results from load_from_str
will work as before.

To ensure consistent behavior for the Node and Yaml I moved methods
from the impl Yaml block to a new trait called YamlNode which is
implemented by Yaml, Node, and YamlMarked. This is a breaking
change since it means that consumers will have to import YamlNode to
use methods like .as_str() and .is_array().

I want to present this pull request as one proposal. I think there is also
an argument for changing the existing Yaml type to incorporate source
location markers instead of maintaining two parallel enums.

While making changes I split up yaml.rs into three nested modules.
I can put it back the way it was if that is preferable.

[hallettj]

So I used the derivative crate to define Hash and PartialEq implementations for Node. Derivative uses procedural macros, which breaks compatibility with Rust v1.24. The reason I did that is that derivative makes it easy to ignore certain struct fields in the hash and equality implementations. Because Node values are used as hash keys, and YAML's hash keys do not differ based on source location, the source location marker in the Node struct should be ignored when comparing Node values.

If you want to maintain compatibility with earlier Rust versions I can make changes to remove derivative, and implement Hash and PartialEq by hand.