utf8 pos cache: always keep most recent value

utf8 pos cache: always keep most recent value

UTF-8 strings may have magic attached that caches up to two byte position
to char position (or vice versa) mappings.

When a third position has been calculated (e.g. via sv_pos_b2u()), the
code has to decide how to update the cache: i.e. which value to discard.
Currently for each of the three possibilities, it looks at what would be
the remaining two values, and calculates the RMS sum of the three
distances between ^ ... cache A .. cache B ... $. Whichever permutation
gives the lowest result is picked. Note that this means that the most
recently calculated value may be discarded.

This makes sense if the next position request will be for a random part of
the string; however in reality, the next request is more likely to be for
the same position, or one a bit further along. Consider the following
innocuous code:

    $_ = "\x{100}" x 1_000_000;
    $p = pos while /./g;

This goes quadratic, and takes 150s on my system. The fix is is to always
keep the newest value, and use the RMS calculation only to decide which of
the two older values to discard. With this fix, the above code takes 0.4s.
The test suite takes the same time in both cases, so there's no obvious
slowdown elsewhere with this change.