Prevent generating duplicate exports for closure conversions

[daxpedda]

We have a report in #4371 where wasm-bindgen would generate multiple exported functions with the same name, which is invalid. Specifically these functions were Closure converters.

I was debugging this just with the generated Wasm module, because I couldn't build the project locally and there was no minimal re-production, the following is just speculation:

The culprit seems to be:

wasm-bindgen/src/closure.rs

Lines 340 to 344 in 88452fa

	#[inline(never)]
	#[cfg_attr(wasm_bindgen_unstable_test_coverage, coverage(off))]
	unsafe fn breaks_if_inlined<T: WasmClosure + ?Sized>(a: usize, b: usize) -> u32 {
	super::__wbindgen_describe_closure(a as u32, b as u32, describe::<T> as usize as u32)
	}

For some reason, while we are interpreting this descriptor, we find multiple ones with exactly the same name, e.g.: <dyn core[bd1a764364e91139]::ops::function::FnMut<(), Output = _> as wasm_bindgen[79ff92f6c8c42bdb]::closure::WasmClosure>::describe::invoke::<()>. While finding multiple ones should not be surprising, as we are dealing with a generic, the name shouldn't be the same because as you can see Rust adds some naming scheme to prevent that. So either this is a bug in Rust, which I very much doubt, or the problem is #[inline(never)], which would not be bug in Rust because the reference clearly states that it is just a suggestion and not a guarantee.

To fix this we simply apply a number at the end to mangle the name further.

Fixes #4371.
Cc @dpc.

[dpc]

I have verified that the version from PR passes wasm-opt problem I originally reported. Will report if find any issues later down the line, but so far LGTM. Thank you so much for help and fix!