Batteries-included proc-macro utils

This re-exports the common libraries - syn, quote, and proc-macro2, that you need to work with proc-macros, as well as utilities to interop between them.

Re-exports

Most of quote, proc_macro, and proc_macro2 are re-exported, and they can be used with the pm:: prefix. Types that have the same name in proc_macro and proc_macro2 are suffixed with 2 in the proc_macro2 export.

Types from syn can be used as with syn:: prefix, by adding the prelude import use pm::pre::*.