bitstring: add constant folding for bitsmod operator
bitstring: add constant folding for bitsmod operator

Implement the reduction for the bitsmod operator where two
constant inputs are being reduced to a constant.

Signed-off-by: Helge Bahmann <c5618920f622561f9c0ec5c42219c4c08180149f@chaoticmind.net>

phate/jive,phate/jive,phate/jive