Add new Data.Bits.Bits(zeroBits) method

This adds a new method to `Bits` which completes the Bits API
with a direct way to introduce a value with all bits cleared.

See also original proposal at

  http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/21157

(cherry picked from commit 83bd2f5fc7e4d748ee35911876c1d4239689db0f)
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>