In addition to invoking js_string() with existing UTF-16 strings when
possible, RegExpExec now takes a Utf16String instead of a Utf16View. The
view was previously fully copied into the returned result object, so
this prevents potentially large copies of string data.
This commit does not go out of its way to reduce copying of the string
data yet, but is a minimum set of changes to compile LibJS after making
PrimitiveString hold a Utf16String.
This implementation closely follows the StringIterator object in that
the abstract closure meant to be created in CreateRegExpStringIterator
is instead unrolled into RegExpStringIterator.prototype.next.
