Use xbyak's std::string interface
Revert "Rename StringPiece to string_view"

This reverts commit 817f6d7ab4a5687825d8ce0b1ae6a3198c14ca94.
Revert "Rename UStringPiece to U16StringPiece and use std::u16string"

This reverts commit d8feeba27ea993eab0a80219eec8b7f4534f2048.