macros: improve 1.0/2.0 interaction

[jseyfried]

This PR supports using unhygienic macros from hygienic macros without breaking the latter's hygiene.

// crate A:
#[macro_export]
macro_rules! m1 { () => {
    f(); // unhygienic: this macro needs `f` in its environment
    fn g() {} // (1) unhygienic: `g` is usable outside the macro definition
} }

// crate B:
#![feature(decl_macro)]
extern crate A;
use A::m1;

macro m2() {
    fn f() {} // (2)
    m1!(); // After this PR, `f()` in the expansion resolves to (2), not (3)
    g(); // After this PR, this resolves to `fn g() {}` from the above expansion.
         // Today, it is a resolution error.
}

fn test() {
    fn f() {} // (3)
    m2!(); // Today, `m2!()` can see (3) even though it should be hygienic.
    fn g() {} // Today, this conflicts with `fn g() {}` from the expansion, even though it should be hygienic.
}

Once this PR lands, you can make an existing unhygienic macro hygienic by wrapping it in a hygienic macro. There is an example of this in the tests.

r? @nrc

[jseyfried]

Déjà vu :)

[kennytm]

Both #46550 and #46419 have been merged.

[kennytm]

Review ping for you @nrc!

[alexcrichton]

ping @nrc, this may be ready for a look now!

[kennytm]

Review ping for you @nrc!

[nrc]

@bors: r+

[bors]

📌 Commit b766fa8 has been approved by nrc

[bors]

⌛ Testing commit b766fa8 with merge 0b90e4e...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nrc
Pushing 0b90e4e to master...