Revisit usize / u64 use for resource mappings in context of index

[nilslice]

Considering some existing code that logically returns values as u64:

qcs-sdk-qir/src/interop/instruction.rs

Lines 137 to 146 in 121101e

	/// Attempt to cast a pointer to an immediate int and return that value if successful
	pub(crate) fn pointer_value_to_u64(
	context: &QCSCompilerContext,
	value: PointerValue,
	) -> Option<u64> {
	value
	.const_to_int(context.base_context.i64_type())
	.get_sign_extended_constant()
	.and_then(|value| u64::try_from(value).ok())
	}

qcs-sdk-qir/src/interop/instruction.rs

Lines 88 to 94 in 121101e

	"Result" => {
	let result_index = pointer_value_to_u64(context, ptr_value)
	.ok_or_else(|| {
	eyre!("unable to derive Result index from pointer")
	})?;
	Ok(OperationArgument::Result(result_index))
	}

These values are regularly re-used elsewhere and need to be cast to usize, which technically can cause truncation, e.g.

qcs-sdk-qir/src/output/debug.rs

Lines 55 to 61 in 82d7339

	#[allow(clippy::cast_possible_truncation)]
	let index = *index as usize;
	if let Some(result) = shot.get(index) {
	output.push(format!("[shot:{} result {}]", shot_id, result));
	} else {
	return Err(Error::NoShotDataAtIndex(shot_id, index));
	}

While it's not a major issue now, it's worth re-visiting in the future. This issue tracks the intent to do so.