Expression ownership (again) and destructors

Had a bug where an expression's type inexplicably became a Function rather than a UDT. Suspected ownership bug.

Had to switch all expression ownership to shared_ptr. Fixing destructors to occur semantically, as is correct, instead of being collected at codegen time. Lots of messy ABI details like type complexity. And I'll have to change AggregateType's functions to be real functions instead of simply complex expressions.

Yay.