Cheap synchronous Rust calls

[janpaul123]

Right now callRustSync is optimized for the case of few calls, with lots of data per call. With Flux we're seeing the opposite pattern: lots of calls with just a few f64 numbers per call.

Currently the overhead consists of:

• Having a function name string for each call. We should probably make a separate wasm export for every function, instead of bundeling it all in one wasm export.
• Creating a buffer with data. When passing in just a few numbers, it'd be nice if we can just pass them directly as function parameters. Separate ticket for this: ZapParam Number #134

For now the workaround is to batch calls to reduce the relative overhead, where possible.