Calls to constrained functions in an unconstrained context are needlessly inefficient

[TomAFrench]

See the mul function below which calls the unconstrained variant and then constrains it using evaluation_quadratic_expression:

noir-bignum/src/fns/constrained_ops.nr

Lines 396 to 412 in b44ef7f

	pub(crate) fn mul<let N: u32, let MOD_BITS: u32>(
	params: P<N, MOD_BITS>,
	lhs: [Field; N],
	rhs: [Field; N],
	) -> [Field; N] {
	let result = unsafe { __mul::<_, MOD_BITS>(params, lhs, rhs) };
	evaluate_quadratic_expression(
	params,
	[[lhs]],
	[[false]],
	[[rhs]],
	[[false]],
	[result],
	[true],
	);
	result
	}

If this mul function is called from an unconstrained context then we need to execute both the __mul function and the evaluation_quadratic_expression which is pointless in an unconstrained context and needlessly slows down execution. We can avoid the need for this by conditioning the evaluation_quadratic_expression on !std::runtime::is_unconstrained() so this only happens in constrained code.