Add field assignment

[CohenArthur]

Closes #191

[CohenArthur]

Since we are mutating an existing instance of "something", we might need to keep track of all instantiated variables in the context in order to mutate them later. This should be done in another PR

[CohenArthur]

We can set a field's value in one of those two cases:

• We just created a variable, and are mutating one of its fields.
• We are mutating an argument given to a function

Thus, we can only keep track of variables to mutate via the scope map. We should be able to use the get_var and replace_var methods already present in it.

The only other problem remaining is to allow the mutation of a field of a field of a field... of a variable. Think vec3.point.x = 15. Which I am unsure how to resolve currently, as I'd like to avoid keeping a Box<dyn Instruction> as value in the FieldAssign struct