Rename stdlib's `bigint` as `altfield`

[Savio-Sou]

Problem

There is currently the bigint.nr library in the standard library suite, which is an implementation of fixed modulo integers sized to a set of elliptic curves (e.g. BN254, secp256k1).

The library is also documented as "Big Integers" here: https://noir-lang.org/docs/noir/standard_library/bigint/

This has been confusing to Noir developers, who often misinterpret it as an arbitrarily-sized large integer type given how that the term "BigInt" is more commonly understood in other programming languages:

• https://github.com/shuklaayush/noir-bigint was "mistakenly" archived susceptibly due to the misinterpretation
• We started developing and maintaining the similarly named BigNum library for arbitrarily-sized large integers
• I anecdotally clarified the confusion with at least three different teams in person

Happy Case

The library and the corresponding documentation should be renamed to something less confusing per general software development intuitions.

I am suggesting altfield but not necessarily feeling strongly about it, open to suggestions.

Project Impact

Nice-to-have

Impact Context

The confusing name have been inducing additional education and ecosystem frictions.

Would you like to submit a PR for this Issue?

None

Support Needs

No response

[Savio-Sou]

bigint was removed as of #7411.