auto merge of #15324 : sneves/rust/master, r=alexcrichton

auto merge of #15324 : sneves/rust/master, r=alexcrichton

The current implementation of `rotate_left` and `rotate_right` are incorrect when the rotation amount is 0, or a multiple of the input's bitsize. When `n = 0`, the expression

    (self >> n) | (self << ($BITS - n))

results in a shift left by `$BITS` bits, which is undefined behavior (see https://github.com/rust-lang/rust/issues/10183), and currently results in a hardcoded `-1` value, instead of the original input value. Reducing `($BITS - n)` modulo `$BITS`, simplified to `(-n % $BITS)`, fixes this problem.