Run jsbeeb mode in less accurate mode and hope it goes faster

[mattgodbolt]

See #47 - we can probably compromise on cycle accuracy in an "overclock" mode and see if that buys us more performance.

Specifically:

• Running more video clocks back to back instead of per instruction.
• Disabling the "cycle accurate" ticking inside instructions.
• Mayyyyybe some magical webasm trickery - see Feature request: jsBeeb and WebAssembly jsbeeb#150