Removed a few #[allow(dead_code)]

[RunDevelopment]

Changes:

• Removed unused nargs field in AuxImport::Closure. The field wasn't used because the generated JS code used ...args to capture any number of arguments. If this information is necessary in the future, AuxImport::Closure already includes the adapter ID of the closure function and the number of args can be retrieved from that.
• Removed unused hole field in Instruction::OptionWasmToStringEnum. The bindings have a comment explaining how the hole is handled implicitly as an "invalid" value, so it doesn't need to know the actual value of the hole.
• Changed how 8/16/32/64 bit ints are converted.

So about the integer conversion: Instead of removing the unused fields, I realized that Instruction::IntToWasm and Instruction::WasmToInt were kind of busted. The main issue is that they not only carried redundant data, but could also carry invalid data.

To fix this, I first split both into 64 and 32 variants. This immediately makes the JS -> WASM conversions nops. The WASM -> JS directions are also simpler, because they just need a single bool of data to select the correct conversion.

This also fixes a minor bug in representable_without_js_glue. The function previously incorrectly determined that WASM -> JS u64 and WASM -> JS NonNull did not require glue code. This was trivial to fix with the new representation.

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This PR should not affect the output of the CLI in any way.

[daxpedda]

LGTM.
Will look into the other unused fields.