Removed messy generics for multiplexers.

Now create a different multiplexer module depending on the
number of its inputs and outputs. Port widths are still generics.

Need to verify mux_4to1 and mux 2to1

 On branch feature/modules
	modified:   src/common_types.vhd
	renamed:    src/rtl/mux.vhd -> src/rtl/mux_2to1.vhd
	deleted:    src/test/mux_tb.vhd