feat!: automatically validate inputs

[asterite]

Description

Problem

Resolves #4218

Summary

This PR transforms a program like this:

fn main(x: u8, y: u8) {
    assert_eq(x, y);
}

into this one:

fn main(x: u8, y: u8) {
    if !std::runtime::is_unconstrained() {
        std::validation::AssertsIsValidInput::assert_is_valid_input(x);
        std::validation::AssertsIsValidInput::assert_is_valid_input(y);
    }
    assert_eq(x, y);
}

The idea is that all inputs now must implement the AssertsIsValidInput trait.

The if !std::runtime::is_unconstrained() check is so that these checks aren't done for brillig (I thought --force-brillig would be passed to the Elaborator to avoid having this check but this wasn't the case).

Pending:

• Validate return values from unconstrained functions called from constrained functions
• Make sure the name of the trait and its method are okay
• Docs

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]

Changes to circuit sizes

Generated at commit: b0e1584992ecc528c7a85dd034339958dbe272f8, compared to commit: 677c10c50e6944e9e11d5579048f77cab59cf91a

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
6	+127 ❌	+57.99%	+4,030 ❌	+56.86%
conditional_regression_short_circuit	+127 ❌	+53.36%	+4,031 ❌	+56.73%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
6	346 (+127)	+57.99%	11,118 (+4,030)	+56.86%
conditional_regression_short_circuit	365 (+127)	+53.36%	11,137 (+4,031)	+56.73%
sha256	1,436 (+157)	+12.28%	24,583 (+4,019)	+19.54%
sha256_var_padding_regression	3,818 (+350)	+10.09%	193,653 (+31,417)	+19.36%
fold_call_witness_condition	5 (+1)	+25.00%	19 (+1)	+5.56%
regression	278 (-2)	-0.71%	3,811 (0)	0.00%
array_if_cond_simple	43 (-59)	-57.84%	2,977 (-148)	-4.74%