auto merge of #11974 : huonw/rust/no-at-vec, r=pcwalton

This removes @[] from the parser as well as much of the handling of it (and `@str`) from the compiler as I can find.

I've just rebased @pcwalton's (already reviewed) `@str` removal (and fixed the problems in a separate commit); the only new work is the trailing commits with my authorship.

Closes #11967

diff --cc src/librustc/metadata/common.rs
Simple merge

diff --cc src/librustc/metadata/decoder.rs
Simple merge

diff --cc src/librustc/metadata/encoder.rs
Simple merge

diff --cc src/libstd/vec.rs
Simple merge