Fix memory leak in symbol table (#209)

Fix memory leak in symbol table (#209)

Commit 4cab39bd4f166716cd3d357a175c346afb838137 moved d_exprMap, d_typeMap,
and d_functions into SymbolTable::Implementation but did not move the deletion
of those objects from SymbolTable to the SymbolTable::Implementation
desconstructor, resulting in a memory leak. This commit fixes the issue.