Reorganize types

[m4rw3r]

Currently Chomp's module-tree is a bit deep for what it does. Would be more ergonomic with a flatter tree.

The most common types should be in the crate root:

• chomp::Input

• chomp::ParseResult (Parser in the case of 2.0)

• chomp::Buffer <- Rename

  Should be renamed, conflicts with chomp::buffer as well as chomp::buffer::Buffer and is more like a window into the token-stream, it is not guaranteed to be a buffer or slice. Maybe Window?