Continuation of #25180. This one refactors the sequence routines.
Preparation for extending with new routines.
Mostly removes repeating code to simplify debugging.
Removes:
- `incrSeqV2` superseded by `incrSeqV3`,
- `setLengthSeq` superseded by `setLengthSeqV2`
Note comment on line 338, acknowledging that implementation of
`setLenUninit` from #25022 does zero the new memory in this branch,
having been copied from `setLengthSeqV2`. This PR does not fix this.