Implement invalidation as a flag check, not a function call.
Some attempts to simplify the generated js code.
Further attempts to simplify the generated asmjs code.
Fix broken array-write-barrier implementation.

It needs speeding up, but at least it works.
Fix output for floatvar initializers.
Directly emit guard-failure checks, rather than not-guard-success checks.

This transforms e.g. !(a==b) into (a==b) in the guard checking code.
Improved constant-folding and other cleanups
Fix accidental zeroing of gcmap at exit.
Fix frame-realloc call to actually use the right depth.