Automatically re-borrow the ExtCtxt in quotes

[kmcallister]

Code like

fn foobar(cx: &mut ExtCtxt) {
    quote_expr!(cx, 1i);
    quote_expr!(cx, 2i);
}

fails to compile because the first quote takes ownership of the &mut ExtCtxt:

macro_crate_test.rs:41:5: 41:25 error: use of moved value: `cx`
macro_crate_test.rs:40:5: 40:25 note: `cx` moved here because it has type `&mut syntax::ext::base::ExtCtxt<'_>`, which is moved by default (use `ref` to override)

The solution is always (?) to re-borrow the ExtCtxt like so:

fn foobar(cx: &mut ExtCtxt) {
    quote_expr!(&mut *cx, 1i);
    quote_expr!(&mut *cx, 2i);
}

The quote macros should do this automatically, for consistency with ordinary Rust functions and to reduce boilerplate.