Add documentation to many parts of the Rust codebase (#552)

* add lots of doccomments

* add conversion traits from layerdatatypes to layers

* add suggested doc improvements

* Code review changes

Co-authored-by: Keavon Chambers <keavon@keavon.com>

graphene/src/lib.rs

@@ -1,8 +1,12 @@
 pub mod boolean_ops;
+/// Contains the [Color](color::Color) type.
 pub mod color;
+/// Contains constant values used by Graphene.
 pub mod consts;
 pub mod document;
+/// Defines errors that can occur when using Graphene.
 pub mod error;
+/// Utilities for computing intersections.
 pub mod intersection;
 pub mod layers;
 pub mod operation;