Try to lift the dependency on unstable rust

[Evelyn-H]

I already attempted to try to use proc-macro-hack instead, but it failed to work, likely because of they weird (and honestly, probably bad) construction it builds to allow the user to use the . operator to access the different sub-parsers / non-terminals:

// user code
let parser = grammar! {
    something = ...
    something_else = ...
}

would essentially expand to this:

let parser = {
        struct PEGParser {}
        impl PEGParser {
            fn something<'input>(&self, input: &'input str) -> ... 
                ...
            }
             fn something_else<'input>(&self, input: &'input str) -> ... 
                ...
            }
        }
    PEGParser {}
    }
}

and then it can be used like this:

let result = parser.something("some string");

An alternative way to (hopefully) circumvent the use of the currently unstable #![feature(proc_macro_hygiene)] could be to do something like this instead:

// user code
mod parser {
    grammar! {
        something = ...
        something_else = ...
    }
}

and then expand to this, without the weird struct/impl construction:

mod parser {
    fn something<'input>(&self, input: &'input str) -> ... 
        ...
    }
    fn something_else<'input>(&self, input: &'input str) -> ... 
        ...
    }
}

and the use would be slightly different, but not much:

let result = parser::something("some string");

This should "just work" on stable rust AFAIK, and in the worst case it should definitely work using proc-macro-hack.

(Note: wrapping the grammar! invocation in a mod is simply to avoid namespace pollution, it could technically be left out if the user wants that.)