Adding support for muxes and flip-flops

[sw23]

Proposing support for multiplexors (muxes) and flip-flops. Here are a few examples:

[[ "x", ["?", "sel", "a", "b"] ]]

[[ "x", [ "?", "s", [ "&", "a", "b", "c", "d", "e" ], "f", "g", "h" ]]]

[[ "x", [ "?", "s1", [ "?", "s0", "a", "b", "c", "d" ], 
                     [ "?", "s0", "e", "f", "g", "h" ],
                     [ "?", "s0", "i", "j", "k", "l" ],
                     [ "?", "s0", "m", "n", "o", "p" ]]]]

[[ "q", [ "$dff", { "d": [ "|", "x", "y" ], "clk": "clk", "s": "set", "r": [ "&", "rst0", "rst1" ], "qn": "q_bar" } ]]]

GitHub Copilot wrote most of the code and I reviewed and had it refactor a few things. Any feedback is welcome!

I followed the proposal in #10
This proposes an implementation for #11