WasmFS code size

[tlively]

Tracking issue for optimizing WasmFS code size. To the extent possible, WasmFS should be pay-as-you-go and should definitely have better code size than the previous system.

[kripken]

Until we optimize for code size various code size checks will not pass (since WasmFS adds enough code by default to break the test assumptions). I'll link those tests to here, so that we can get back to them later.