FJCVTZS (Floating-point Javascript Convert to Signed fixed-point,
rounding toward Zero) does exactly what we need for ToInt32 in the
JavaScript specification.
This isn't world-changing, but it does give a ~2% boost on compute-
heavy benchmarks like JetStream, so we should obviously use it.
The fast path of to_i32() can be neatly inlined everywhere, and we still
have to_i32_slow_case() for non-trivial conversions.
For to_u32(), it really can just be implemented as a static cast to i32!
