sully_input: Rich input for parsers

This is a helper library for the sully_peg parser generator proc macro.

It exposes the following functionality:

• Structs Input and Span parameterized on the lifetime of a string slice.
• Terse debugging info for each structure (useful for embedding in tree nodes).
• Trait Exact to allow type-defined parsing for structures with exact string representations (such as single characters or strs).