WASM compilation error due to loop unrolling

[paberr]

It seems that the loop unrolling on the Fp::into_bigint function results in an excessive number of local variables.

@ii-cruz and I have created a minimal example showcasing this problem: https://github.com/paberr/arkworks-example/blob/main/tests/test.rs

Steps to reproduce

1. Checkout https://github.com/paberr/arkworks-example
2. Install wasm-pack
3. Run wasm-pack test --node

This will fail with the following error:

Error: failed to deserialize wasm module

Caused by:
    0: failed to parse code section
    1: locals exceed maximum (at offset 16377)

Troubleshooting

This error indicates that a function tries to allocate more than 50,000 local variables.
This problem initially occurred in a larger codebase and we traced it back to the into_bigint function.
Using the minimal example, the behaviour is reproducible.

We initially suspected that this happens due to excessive code inlining.
However, using a patched version of ark-ff, we found that the inlining does not trigger the error.
It is the loop unrolling, which deterministically creates the error.
Removing or reducing the unrolling will make the problem disappear.