factor out SymbolTablMixin

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
refactor

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
PopNested initial

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Implement PopNested

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>