Add support for parsing bound expressions. Still does not implement them
Remove input_ident parameter from generate_parser. It will always be 'input'
Use a type def for the return type of parses and deduplicate parser mod codegen in lib
Update Cargo.toml according to latest cargo build
Move some logic from lib.rs into a new function, back::generate_parsers
Move most of the logic from generate_parser_expr into separate functions
Make all parser generation functoins take an immutable reference of ExtCtxt instead of a mutable one
Add a Compiler type to back and make all parser generation functions methods for that
Move remainder of codegen from lib into back::Compiler::compile
Add a note about what kind of values lookheads yield