Fixes and cleanups in the yacc parser

This introduces various memory leak fixes and cleanups in the parser. It additionally enables a valgrind run with check for memory leaks.

It does change the generated structures by removing non-functional nodes.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with other code
• This feature/change has adequate documentation added
• No obvious mistakes in the code