Better const field and group initialization

[Pratyush]

Description

Leverages improved Rust support for const fn to enable better initialization of field and group elements in the const contexts.

Examples include:

// old
const f: Fp = MontFp!(Fp, "123");
// new
const F: Fp = MontFp!("123");

// old
const F2: Fp2 = QuadExt!(F, F);
//new
const F2: Fp2 = Fp2::new(F, F);

// old, impossible:
const GENERATOR: G1Affine = ???;
// new
const GENERATOR: G1Affine = G1Affine::new_unchecked(X, Y);
// ^ we use unchecked here to avoid on-curve and subgroup-membership tests

Closes #296

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• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[mmagician]

Looks good to me now!

[swasilyev]

It seems CHANGELOG hasn't been updated