`self` variables are scoped to methods, modules, classes and the
top-level of the program. Prior to this change, they were treated as
being scoped just to methods.
This change means we (once again) correctly synthesise `self` receivers
for method calls in class bodies, module bodies and at the top-level.
We can safely create uninitialized writes for `self` variables, because
they appear at index -1 in the entry block of a method, and are
immediately overwritten by a write to `self` at index 0. As a result,
they are not live and will be pruned from the CFG.
We model `self` variables by inserting a write at the start of every
method body. We then treat them as local variables that are alive for
the extent of the method body.
