Redefinition bug for structs

[mark-koch]

The following raises a KeyError:

module = GuppyModule("test")

@guppy.struct(module)
class Foo:
    x: int

@guppy(module)
def foo() -> int:
    return y  # Deliberate error

try:
    module.compile()
except:
    pass

@guppy.struct(module)
class Foo:
    x: int

module.compile()