Allow crate-relative imports in grammars

[dario23]

fixes #212

[kevinmehall]

Thanks!

Updated it to fix a few things:

• Changed the rule to ("crate" "::")? IDENT() ++ "::", using ? to simplify the optional part, and splitting "crate::" into two tokens because use crate :: foo is valid Rust syntax. (this is a feature of the implementation of the Parse traits for TokenTree used in the macro; when writing a grammar for &str they'd be equivalent).
• Added a #[derive] needed to make the test compile
• Updated the compiled grammar with ./bootstrap.sh -- I'll add a development guide to the readme.