Better playground, and some tweaks to runtime to support it

- Print errors with toString
- Print using toReprJS
- Actually show errors
- Allow configurable gas, async
- Pause and stop buttons
this call was never necessary
these should have been with 2 commits ago; new calling convention for run()
this parses as a name
more new run() calling convention
dead code
This was a bad attempt at using stack frames better

the increment here was incorrect, and could cause programs with
complex expressions in body positions to consume stack that wasn't
accounted for by GAS, causing stack overflows
forgotten A
include main wrappers