Allow building without proc_macro dependency

[dtolnay]

@golddranks this lets you disable the proc_macro dependency:

proc-macro-hack = { version = "0.3", default-features = false }

The only catch is that then your crate is responsible for providing ::proc_macro, but as I understand it this should work with musl target.

extern crate proc_macro;

#[macro_use]
extern crate proc_macro_hack;

proc_macro_expr_impl! {
    /* ... */
}

Fixes #6.