Allow `pat` macro matcher to be followed by `:`

[GrantMoyer]

Similarly to #1336, since function parameters can be destructured with patterns:

fn foo((x, y): (u32, u32)) -> u32 {
    …
}

pat fragment specifiers should be able to be followed by :. The motivation case for this is something like:

macro_rules! terse {
    ($name:ident($($arg:pat : $type:ty),*) $body:block => {
        fn $name<T>($($arg : $type),*) -> T
            where really::long::and::tedious::Bound<That=Even, Has=Associated> + Types
        $body
    }
}

terse!{bar((x, y): (u32, u32)) {
    …
}}

[ExpHP]

Alternatively there is an existing proposal to make pat: ty a valid pat.

(and such considerations were the reason why : was chosen not to be allowed after pat)