Get rid of memory leaks in test code.

[svhawkins]

Have fun with valgrind.

[svhawkins]

12.02.2023::00:41:

• Test code (test_stack, test_typecheck, and test_codegen, got their memory leak problems resolved. There are still more frees than allocations, but better to be safe than sorry. test_codegen needed an additional fclose(), test_typecheck got its tests modified, either to be simpler or due to other changes that I made (symbol table not destroying allocated table at scope exit and inclusion of hidden symbols). test_stack had problems with hidden symbols.
• test_ast still has a memory leak, but is traced to bison-generated functions that are outside of my jurisdiction (unless I make my own PDA).

[svhawkins]

Resolved in latest commit, test_stack and test_typecheck abort due to illegal free when running all tests or running without valgrind.

10 addresses this issue. Memory problems now are hidden symbol centered.

[svhawkins]

Issue #10 resolved the memory issues (so far). The big culprit was the hidden symbol implementation. It has been removed. A better implementation for it will soon come, however.