Add debug utilties

This adds a few extra commands to the nextpnr and yosys passes which I needed to debug some poor resource usage. For yosys, you can now specify a custom synthesis script, like

proc
flatten
tribuf -logic
deminout
opt_expr
opt_clean
check
opt -nodffe -nosdff
fsm
opt
wreduce
peepopt
opt_clean
share
show -prefix build/graph -format dot -colors 0

Which performs synthesis all the way up until mapping to luts, then shows the result.

For nextpnr, the change is much simpler, just a flag to enable the GUI

Both of these are only useable when running swim pnr/synth directly. This ensures that downstream flows don't have issues if the custom stuff doesn't do what we expect.