Keep the legacy regexp static properties backed by PrimitiveString
values instead of eagerly copied Utf16Strings. lastMatch, leftContext,
rightContext, and $1-$9 now materialize lazy Substrings from the
original match input when accessed.
Keep RegExp.input as a separate slot from the match source so manual
writes do not rewrite the last match state. Add coverage for that
behavior and for rope-backed UTF-16 inputs.
0969a5cd9a