implement function name storage and its respective instructions

[joao-novo]

This is fairly easy, all that is required is:

• Create a symbol table type for functions
• Update the table accordingly
• When a function is called, look for it in the table, and if it's not found, return an error
• Replace the function call with its respective instructions (might not be necessary)

[joao-novo]

postponed for now as this won't be useful until later and this will require state monads, which i don't currently know how they work