macros: improve 1.0/2.0 interaction
This PR supports using unhygienic macros from hygienic macros without breaking the latter's hygiene.
```rust
// 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](b766fa887d) of this in the tests.
r? @nrc
0b90e4e8cd