Make Scalar conversions explicit

[maujim]

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• The PR title and commit messages adhere to guidelines here: https://github.com/spaceandtimelabs/sxt-proof-of-sql/blob/main/CONTRIBUTING.md. In particular ! is used if and only if at least one breaking change has been introduced.
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Rationale for this change

• this is step 1 of 3 for Make opaque Scalar conversions explicit rather than using From/Into trait. #228, each step will ideally be a standalone PR
• /claim Make opaque Scalar conversions explicit rather than using From/Into trait. #228

What changes are included in this PR?

Copied from description of #228

• Remove Into<[u64; 4]>, From<[u64; 4]>, and RefInto<[u64; 4]> bounds.
• Replace these with the trait methods fn from_limbs(val: [u64; 4]) -> Self and fn to_limbs(&self) -> [u64; 4]

Are these changes tested?

Not yet, thats why its in draft

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[maujim]

Ok so I have it mostly working except for this one compilation issue. At the latest commit, I'm getting multiple errors of the following format in owned_column_operation.rs:

error[E0277]: the trait bound `num_bigint::BigInt: base::scalar::scalar::Scalar` is not satisfied
   --> crates/proof-of-sql/src/base/database/owned_column_operation.rs:595:21
    |
593 |                 Ok(Self::Boolean(ge_decimal_columns(
    |                                  ------------------ required by a bound introduced by this call
594 |                     lhs_values,
595 |                     rhs_values,
    |                     ^^^^^^^^^^ the trait `base::scalar::scalar::Scalar` is not implemented for `num_bigint::BigInt`
    |
    = help: the following other types implement trait `base::scalar::scalar::Scalar`:
              MontScalar<ark_bls12_381::FrConfig>
              MontScalar<ark_curve25519::FrConfig>
note: required by a bound in `ge_decimal_columns`
   --> crates/proof-of-sql/src/base/database/column_operation.rs:686:8
    |
679 | pub(super) fn ge_decimal_columns<S, T>(
    |               ------------------ required by a bound in this function
...
686 |     S: Scalar,
    |        ^^^^^^ required by this bound in `ge_decimal_columns`

I'm not sure how to proceed from here. We could try adding a impl Scalar for BigInt block. I gave it a quick spin and the only thing I could not figure out was how to implement Scalar.MAX_SIGNED using BigInt. @JayWhite2357 please let me know if you any suggestions here.