Get rid of bounds check in slice::chunks_exact() and related functions during constru...

Get rid of bounds check in slice::chunks_exact() and related functions during construction

LLVM can't figure out in

    let rem = self.len() % chunk_size;
    let len = self.len() - rem;
    let (fst, snd) = self.split_at(len);

and

    let rem = self.len() % chunk_size;
    let (fst, snd) = self.split_at(rem);

that the index passed to split_at() is smaller than the slice length and
adds a bounds check plus panic for it.

Apart from removing the overhead of the bounds check this also allows
LLVM to optimize code around the ChunksExact iterator better.