Change the field `function` from an `Expression` to an `Identifier` in `CallExpression`

[mohammadfawaz]

There's already a TODO there to fix that. Since no other expression can represent a function, it's safe for the parser to reject any expression that is not an identifier. This makes extracting the name of the function called much easier in later compiler phases.

[tetsuo-cpp]

I'm interested in hacking on Leo and would like to give this issue a try.

[mohammadfawaz]

@tetsuo-cpp you're welcome to! Please let me know if you have any questions.