libcore: add borrow module

Following [the collections reform
RFC](https://github.com/rust-lang/rfcs/pull/235),
this commit adds a new `borrow` module to libcore.

The module contains traits for borrowing data (`BorrowFrom` and
`BorrowFromMut`),
generalized cloning (`ToOwned`), and a clone-on-write smartpointer (`Cow`).
libcollections: deprecate _with methods

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

[breaking-change]
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.