chore: remove the as_field and from_field built-ins

[asterite]

Description

Problem

Resolves #6838

Summary

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Peak Memory Sample

Program	Peak Memory
keccak256	78.57M
workspace	123.47M
regression_4709	423.68M
ram_blowup_regression	1.58G
private-kernel-tail	207.24M
private-kernel-reset	720.82M
private-kernel-inner	292.44M
parity-root	172.24M

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.478s	9%
regression_4709	0m0.833s	7%
ram_blowup_regression	0m14.716s	3%
rollup-base-public	3m25.431s	-2%
rollup-base-private	3m2.519s	0%
private-kernel-tail	0m1.225s	-19%
private-kernel-reset	0m8.478s	12%
private-kernel-inner	0m2.675s	10%
parity-root	0m1.155s	20%
noir-contracts	2m55.161s	1%

[github-actions]

Execution Sample

Program	Execution Time	%
sha256_regression	0m0.622s	4%
regression_4709	0m0.395s	0%
ram_blowup_regression	0m4.478s	2%
rollup-base-public	0m28.769s	-2%
rollup-base-private	0m26.523s	1%
private-kernel-tail	0m0.739s	9%
private-kernel-reset	0m1.489s	-2%
private-kernel-inner	0m1.022s	0%
parity-root	0m0.528s	2%

[asterite]

Aztec-Packages is failing now, but I think this uncovered a bug. There's code that does something like this:

fn main() {
    let u = U128::from_integer(184467440737095516152);
    let y = U128::from_integer(u);
    println(y);
}

That is, it's calling U128::from_integer(x) with x being U128 itself. That seems to work in master but just truncates the U128 value. Just using x here is fine.

[TomAFrench]

Looks good to me with a private trait (although in reality it's not really private until we require traits to be in scope in order to use its methods).

[asterite]

Rust has, in a crate, an AsPrimitive trait that's essentially the as keyword but in a programmatic way. In the end that's what as_field and from_field end up doing, and this PR so far too. I just tried introducing such trait and using it, and it works fine. The benefits are that it's just one trait, it's a more general trait, and it could also be a public trait. What do you think?