Document pdep / pext primops
Reference implementations are adapted from https://hackage.haskell.org/package/bitvec-1.1.3.0/src/src/Data/Bit/PdepPext.hs. See also https://hackage.haskell.org/package/bits-extra-0.0.2.3/docs/src/Data.Bits.Pdep.Slow.html#SlowPdep and https://hackage.haskell.org/package/bits-extra-0.0.2.3/docs/src/Data.Bits.Pext.Slow.html#SlowPext for another source.