Symbol, Symbol Test: Drop logger completely; remove unused imports.
Symbol: Simplify lookup_symbol.
Symbol: Inline _find_name_in _current_scope into lookup_symbol.

* Assert correct nesting.
* Handle case where identifier is not present.
* Adapt insert_symbol.
Symbol test: Basic tests.
Symbol: Hash table should be private.
Symbol: Scope is public again.
Symbol test: Test Scope.
Symbol test: Fix tests; test scope visibility.