Removed un-used and broken testbench
added a very generic multiplexer and simple testbench

Requires visual inspection currently. No automatic assertions at present
 On branch feature/modules
 Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)

	new file:   src/common_types.vhd
	new file:   src/rtl/mux.vhd
	new file:   src/test/mux_tb.vhd
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