[Tree Optimizer] Add a function environment to the interpreter

[Alex-Fischman]

Right now the interpreter and typechecker panic/do a bad job when run on functions and function calls. We should fix this.

[oflatt]

I think that the interpreter should run on new thing called a Program which is a list of functions. The first function will be the "main" function That way I can translate Bril to a Program

[oflatt]

Fixed in new schema