Store data type size in the symtable

[MatthewKosloski]

Currently, when creating the stack, we multiply the number of temporary variables used by 8 bytes (see here). It would be better if the stack size was dynamically calculated instead of statically hard-coding the 8-byte multiple. Also, in the future, we may support additional data types that require more space. Thus, it would be better if the symbol table stored data type size information (see page 970 of the Dragon Book).