Subset port of ref/hare/crypto/math/bits.ha: the 32-bit rotations
sha256's message schedule and compression need. The wider bits.ha
surface (rotl64/rotr64, the constant-time compare family, xor) lands as
callers arrive. rotr32 is exercised end-to-end by the sha256 NIST
digest vectors, so no standalone @test ships here.
