Simplify JSString architecture

Use ConsString, SeqString, ExternalString, SlicedString.
Quickly give up picking n's character from String when String is not flat.
Remove sign/unsign comparisons
Check empty range before passing it to mie::findStr

Since string_view pass [nullptr, nullptr) range, it raises segv
in mie::findStr.
Merge branch 'string-rope-simple' of github.com:Constellation/iv into string-rope-simple