`wasm_gen` macros should support conditionals inside them

[pufferfish101007]

Currently if there is a small section of WASM that needs to be conditionally generated (depending on flags, input types etc), it uses the pattern of wasm![...].into_iter().chain( if ... { wasm![...] } else { wasm![...] }).chain(...).collect(), which is fine but not amazing. Ideally we'd be able to write something like

wasm![
    I32Const(0),
    ...,
    @if(condition),
    ...
    @else,
    ...
    @endif,
    ...
]

which would desugar to iterator chaining.

The special instructions (@isnan, @box etc) should then also use this sort of pattern rather than if/else-ing the entire sequence of instructions with a strange bitmask, as I don't really know how the code works at the moment.