wasm32: define c_uint_fast16_t as u32

[egasimus]

I've approached SimplicityHL by compiling the SimplicityHL compiler to wasm32-unknown-unknown, so that I would be able to invoke it from JS.

After producing a program's P2TR address, the next step is to construct a spend PSET; trying to do that, I kept running into an opaque JetFailed error when calling satisfy_with_env.

I traced that to the UWORD size/align checks in simplicity::ffi::c_jets::sanity_checks. Defining c_uint_fast16_t it as u32 on WASM seems to fix it.

[apoelstra]

This value was originally set in commit 0386d3c in #204. The PR description there explicitly says that this value should be u16 not u32.

cc @uncomputable do you remember anything about this, what toolchain you used, etc?

Meanwhile, @egasimus I'm going to need you to write up a bunch more details about what you're doing, so that when we inevitably need to change this again we'll have more information.