Use proper types for values returned from unconstrained functions

[TomAFrench]

There's a lot of cases where Zac has broken out of the type system for zero benefit. This just harms readability without improving performance, e.g. decompose is doing a bit decomposition but return in the values as Field

noir_json_parser/src/_string_tools/slice_packed_field.nr

Lines 589 to 601 in d0fdc8d

	/**
	* @brief converts a 16 bit value into 16 fake bools (Field elements that are 0 or 1)
	**/
	unconstrained fn decompose(val: Field) -> [Field; 16] {
	let mut r: [Field; 16] = [0; 16];
	let mut it = val as u32;
	for i in 0..16 {
	r[i] = (it & 1) as Field;
	it >>= 1;
	}
	r
	}

Rather than using Noir's automatic constraining of return types of unconstrained functions, he's instead injecting a manual boolean check here:

noir_json_parser/src/_string_tools/slice_packed_field.nr

Lines 612 to 613 in d0fdc8d

	// we check that the path valid bits are binary
	assert(path_valid_bits[i] * path_valid_bits[i] - path_valid_bits[i] == 0);

This is a direct 1:1 with what Noir would do if it was just written with the expected types from the start.

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Having written this out, I noticed that Zac is just doing to_le_bits here so we can remove this function entirely.... That said, there's likely a bunch of different cases of this around the library.