Simple fallback if we don't match emphasis.

The other approach led to wrong results on:

    *hi _there*
Removed memoization code.
Used last_emphasis_closer to avoid unneeded scans for closer.

This doesn't seem to help much.
Restored memoization code.
Only memoize during inline parsing.

This cuts the performance hit.

With memoization, we get roughly constant behavior in the fuzztest.
Without it, not.