bitstring: reimplement sext and zext methods with the help of concat method
bitstring: reimplement sext and zext methods with the help of concat method

Reimplement sext and zext with concat method and make them const.

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

phate/jive,phate/jive,phate/jive