Made `gfx_macros`'s macros no longer depend on a `gfx` module in the top
level.

This is achieved by inserting `extern crate gfx;` imports at appropiate
places during expansion.

These imports use unique generated identifiers, and thus don't
conflict with each other or user code.

This also changes gfx-test to not having a `gfx` module in scope, thus
ensuring no new breakage sneaking in.