refactor flisp compiler to handle internal define much more cleanly
WIP: get new flisp closure representation working
clean up flisp closure changes

- remove setc opcode; no longer needed
- add box and box.l for more efficient heap-allocated bindings
- `captured` flag is no longer used, so we can shrink frames by one slot
- remove dead code in VM
flisp optimization: inline lambdas in head position (i.e. `let`)

not a massive improvement, but does make the front end consistently
around 10% faster.
a couple more small front-end speedups