feat: add generic witness reader

[erhant]

hi! I am trying to read witness files in another project of mine (circomkit-ffi) and I came across your witness reader function. I have made the following changes:

• a bit of docs added / refactored
• have the witness parser take a chunk-mapper argument as well, of type impl Fn(&[u8]) -> T
• implement parse_witness_to_bigints by simply calling the generic function with chunk-mapper as BigInt::from_le_bytes
• use match instead of if-else; maybe opinionated but I think it looks more clear here and especially fixes the hidden case of "section 1 pos mutations happen within the else branch of section_id == 2 check" case

with this generic function we are able to read raw witness files into other backends like:

• Arkworks: parse_witness_to_elems(&wtns_data, F::from_le_bytes_mod_order)
• Lambdaworks: parse_witness_to_elems(&wtns_data, |bytes| FrElement::from_bytes_le(bytes).unwrap())

[erhant]

changes added! 🙏🏻

[vivianjeng]

Thank you for the contribution!
It looks good to me

Could you please format the code with

cargo fmt --all

Thank you

[erhant]

hey! fixed with cargo fmt --all and also fixed a few lint issues from cargo clippy 🙏🏻

[vivianjeng]

@erhant I published a new version
please check: https://crates.io/crates/witnesscalc-adapter