Make Bitv::eq_vec take ~[bool] rather than ~[uint]

Fixes #7711.
Replace owned with borrowed pointer.

As pointed out by cmr, there's no need for it to be owned there.
That was also in the original scope of #7711.
auto merge of #7730 : chris-morgan/rust/bitv-eq_vec-bool, r=cmr

Fixes #7711.