cleanup simple_uart.v (remove warnings)

effort levels seem high by default:
    Overall effort level (-ol):   High 
    Router effort level (-rl):    High 
    -> 'make quicksynth' ?

add support for post-synthesis simulation

autoimpact (needs testing)
    impact -mode bscan -b build/sp605.bit -port auto -autoassign (needs testing)

Add linting to tests (aka, tb/*_tb.v)

'lint' should use vfiles, not -I./hdl

add 'S' to the end of netlists ("save") to squelch lots of warnings?

refactor:
  - everything as a /shared/ directory; could be usable as a git submodule or
    just pulling everything in
  - ./sim/ instead of ./tb/