Ensure cell indices are sorted in recover_cells_and_kzg_proofs

[jtraglia]

This PR will reject inputs to the recovery function if the cell indices are not in ascending order. With this check, we remove the fr_is_null duplicate checks below. I've kept the memcpy change because I prefer how the new version reads.

See also the specifications update related to this:

• Ensure cell indices are sorted in recover_cells_and_kzg_proofs consensus-specs#4519

Thanks to @asanso for pointing this out.

[asn-d6]

I see why this is a bit nicer than the blind memcpy, but IMO it's also a bit confusing.

It's confusing because at this point we are past all the initial checks and we also have num_cells == CELLS_PER_EXT_BLOB, and hence we MUST be certain that cell_indices = [0, 1, ..., CELLS_PER_EXT_BLOB-1].

The problem is that this new code introduces index and sorta implies it's a free ranging index, whereas in reality it should be very very constrainted.

For me, the following would be more descriptive:

Suggested change

	for (size_t i = 0; i < CELLS_PER_EXT_BLOB; i++) {
	uint64_t index = cell_indices[i];
	recovered_cells[index] = cells[i];
	for (size_t i = 0; i < CELLS_PER_EXT_BLOB; i++) {
	/* At this point we know that all cells are in the right order */
	assert(cell_indices[i] == i);
	recovered_cells[i] = cells[i];

[asn-d6]

Tried this after our discussion. It does not use an assert but it also does not introduce a new index: 65ebfdd

[asn-d6]

LGTM! Thanks!