we rely, perhaps a bit hackily, on there only being one copy of
each type object floating about. using `deepcopy` on a Message
for message extensions destroys this paradigm, which breaks
things in the case where it's a later defined subtype that contains
variable-length members.
to fix this, we modify `__deepcopy__` on the Field class, such
that it preserves the reference to the original type_obj instance.
Add a test for checking that the bolt-gens do the right thing
for a fairly exhaustive test case set (and that it compiles).
Note that this doesn't check that we've got the memory assignment
pieces worked out.
It's got a kind of exotic reliance on the update-mocks in that in
order to depend on as little of the wire/ code as possible (we
only import wire/wire.h), we include an AUTOGENERATE comment
in the test_cases CSV file, and then run update-mocks as part of
the build for that file.
lisa neigut