rb_str_{partition,rpartition}_m: Handle /\K/ in pattern

When the pattern given to String#partition and String#rpartition
contain a /\K/ (lookbehind) operator, the methods return strings
sliced at incorrect positions.

```
# without patch
"abcdbce".partition(/b\Kc/)  # => ["a", "c", "cdbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcd", "c", "ce"]
```

This patch fixes the problem by using BEG(0) instead of the return
value of rb_reg_search.

```
# with patch
"abcdbce".partition(/b\Kc/)  # => ["ab", "c", "dbce"]
"abcdbce".rpartition(/b\Kc/)  # => ["abcdb", "c", "e"]
```

As a side-effect this patch makes String#partition 2x faster when the
pattern is a costly Regexp by performing Regexp search only once,
which was unexpectedly done twice in the original implementation.

Fixes [Bug #17119]

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