In particular:
- Replace use of "item" with "matcher position/"mp".
- Replace use of "repetition" with "sequence".
- Replace `ms` with `matcher`.

Currently, matches within a sequence are recorded in a new empty
`matches` vector. Then when the sequence finishes the matches are merged
into the `matches` vector of the parent.

This commit changes things so that a sequence mp inherits the matches
made so far. This means that additional matches from the sequence don't
need to be merged into the parent. `push_match` becomes more
complicated, and the current sequence depth needs to be tracked. But
it's a sizeable performance win because it avoids one or more
`push_match` calls on every iteration of a sequence.

The commit also removes `match_hi`, which is no longer necessary.

This avoids some allocations.