Converting to and from smart enums and their corresponding native value
requires knowing the conversion function, since each smart enum has its own
conversion function.
The dynamic registrar can do this at runtime, but it's something that can be
computed in the static registrar at build time, so do that. The static
registrar will now embed the metadata token (as a token reference) in the
generated code, which can be used at runtime to find the conversion method,
without requiring anything from dynamic registrar.
This requires some refactoring in the conversion methods to allow passing more
information around (the `context` parameter in the conversion functions, which
the smart enum conversion methods now use for the token reference).
It turns out this is a massive improvement (although the scenarios where this
is relevant are rare): a simple test case [1] is 300x faster (from ~30s to
0.1s).
[1]: https://gist.github.com/rolfbjarne/961af28d1c2727b256fbcdfd6aa65e98
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