Implemented count, nth, and last in constant time for Windows, Chunks,
and ChunksMut created from a slice.
Included checks for overflow in the implementation of nth().
Also added a test for each implemented method to libcoretest.
Addresses #24214
These implementations were intended to be unstable, but currently the stability
attributes cannot handle a stable trait with an unstable `impl` block. This
commit also audits the rest of the standard library for explicitly-`#[unstable]`
impl blocks. No others were removed but some annotations were changed to
`#[stable]` as they're defacto stable anyway.
One particularly interesting `impl` marked `#[stable]` as part of this commit
is the `Add<&[T]>` impl for `Vec<T>`, which uses `push_all` and implicitly
clones all elements of the vector provided.
Closes#24791
A slice iterator is isomorphic to a slice, just with a slightly
different form: storing start and end pointers rather than start pointer
and length. This patch reflects this by making converting between them
as easy as `iter.as_slice()` (or even `iter[]` if the shorter lifetime
is ok). That is, `slice.iter().as_slice() == slice`.
These are like the existing bsearch methods but if the search fails,
it returns the next insertion point.
The new `binary_search` returns a `BinarySearchResult` that is either
`Found` or `NotFound`. For convenience, the `found` and `not_found`
methods convert to `Option`, ala `Result`.
Deprecate bsearch and bsearch_elem.
