TOPLEVELTYPES is not intended to be configurable by the user, rather is a reflection
on the types defined by the source. This changes the TOPLEVELTYPES argument to be
a property, allowing it to be constructed as a generator expression when generating
BF files. By being a property, and not something like a global property, it removes
the need to ensure that additions to TOPLEVELTYPES are done prior to any bitfield
target definitions.
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