Document Bigint to figure out if we can remove it

[mimoo]

https://github.com/zksecurity/noname/blob/10a24bdc2078371e5f52f8cf5c266fcbd9b4d41e/src/parser/types.rs#L184

I can't remember why we had this type, it would be good to spend some time tracking it through the codebase and document exactly what is its purpose. This way we can decide if we want to remove it, or refactor things.

[ronantakizawa]

BigInt is used in the code to handle very large signed integers that can't be represented by standard fixed-size integer types.

Perhaps we could use Fixed-Size Integer Types or Fixed-size finite field elements?

I was able to replace BigInt with a Field type and have a PR ready for that.

[mimoo]

I believe the PR mentioned above deletes it :o

[ronantakizawa]

The above PR looks good 👍