Support Rust 2018 style imports

[yvt]

This pull request makes parse-generics-shim compatible with Rust 2018 style imports. The introduced changes are twofold:

• It adds local_inner_macros to macro definitions so that they can be imported with Rust 2018 style imports. This is demonstrated in tests/parse_constr.rs.
• Callback parameters can now refer to pathed macros (cratename::aaa:bbb!).

As it stands, use-parse-generics-poc appears to be incompatible with any compiler version that is compatible with Rust 2018 style imports. The only way to use use-parse-generics-poc now is to grab a very ancient version of nightly, which is very unlikely for anyone to do these days. Thus, I think it's acceptable to ignore use-parse-generics-poc for now, at least until it's rewritten as modern procedural macros.