Upgrade libuv to the current master (again)

This is a reopening of the libuv-upgrade part of #8645. Hopefully this won't
cause random segfaults all over the place. The windows regression in testing
should also be fixed (it shouldn't build the whole compiler twice).

A notable difference from before is that gyp is now a git submodule instead of
always git-cloned at make time. This allows bundling for releases more easily.

Closes #8850
auto merge of #8994 : alexcrichton/rust/upgrade-libuv, r=brson

This is a reopening of the libuv-upgrade part of #8645. Hopefully this won't
cause random segfaults all over the place. The windows regression in testing
should also be fixed (it shouldn't build the whole compiler twice).

A notable difference from before is that gyp is now a git submodule instead of
always git-cloned at make time. This allows bundling for releases more easily.

Closes #8850