Improve classification of external symbols

[RaoulHC]

Currently SExternal is a constructor in the symbol table, but it's not actually constructed anywhere even though it's checked for in the constant propagation and type checking. We should try and catch any external statements and make sure the SymbolTableEntry is updated to use SExternal.

For functions it might be worth catching any function calls to a symbol, and then updating the entry accordingly, and for subroutines we'll want to update the to extType to Maybe Type so we can catch them too. They're currently not in the SymbolTable at all, but they are symbols and that can be useful information.