Walk through capturer's parent link using information from symtables.

Current capturer is "self". "parent" link is field of capturer so we can search in symtable and then get type of next capturer. Then generate load and so on. Such code will be more portable than previous version which walks through symtable stack.
Test closure with nested function which captures data.
Fix nesting calculation.
Fix problem with multiple access to same captured variable.

Problem: in case of capturing with deep nesting level(>1) works only first access.
Cause: during first access to captured variable we add copy of this variable to capturer, but symtable of capturer is in symtablestack(see "handle_possible_capture"). Next access will create access to field of capturer which is not handled by us.
Fix: mangle names of symbols which are moved to capturer.
Tests. Closure prodeces closure. More cases of capture.