Add 4 way 1-bit multiplexor
Add 4-way 16-bit multiplexor
Change implementation of 4-way 16-bit multiplexor to use three 2-way 16 bit multiplexors.

It turns out that building the 4-way 1-bit multiplexor first was
working, but rather than combine those up to 16-bit-ness, we can
just combine already existing 16-bit multiplexors to cope with 4
inputs, just as in a previous commit to make an 8-way OR
Add an 8-way 16-bit multiplexor