Does `Program::declarations` need to change?

[asaites]

While merging quil-py into quil-rs, I came upon an old TODO without an issue, which I've moved into quil-rs as so:

    // TODO: (9b9690d4 Marquess Valdez 2023-06-01 14:10:25 -0700) 
    // Should this filtering move to Program? Should we assume memory_regions will always make up all
    // declarations and simplify this?

I'm not totally sure what this comment is getting at, but I wanted to connect the comment to an issue for discussion and tracking purposes.

If the comment is simply outdated, we should remove it and close this issue.

[antalsz]

Yeah, it looks like Marquess was right here! This function is needlessly reconstructing the full set of Quil instructions before pulling out the DECLAREs – but we've already done that! It should work like fn gate_definitions, where it starts self.as_inner().memory_regions.map(…) instead.