This refactors SelfApplyExpr.getFunction() as MethodLookupExpr().getMethodRef().
This is simpler, because we are not hiding DeclRefExprs or reinventing hidden AST resolution.
In order to distinguish overloads of the constructor and for consistency
with other function calls, `ConstructorDecl` string representation uses
the name which includes parentheses and parameter labels.
For consistency also the destructor got the same change, which means
all `DestructorDecl`s will now show as `deinit()` rather than `deinit`.
