libstd: Deprecate _equiv methods

This commit deprecates the `_equiv` family of methods on `HashMap` and
`HashSet` by instead generalizing the "normal" methods like `get` and
`remove` to use the new `std::borrow` infrastructure.

[breaking-change]
Fallout from deprecation

This commit handles the fallout from deprecating `_with` and `_equiv` methods.
libcollections: generalize BTree* to use BorrowFrom

Generalizes the BTree-based collections to use the new BorrowFrom
infrastructure for more flexible lookups and removals.