Export type aliases in LLBC

[msprotz]

This is particularly useful for Eurydice. I have a mechanism for picking user-provided type abbreviations, but instead end up generating things like this https://github.com/cryspen/hacl-packages/pull/457/files#diff-a94673428367ed62df2e7dd7d29ef4b6576ee2bd5e9cf33beed83e3f452e3602R106-R115 because I no longer have the information that the user had provided a custom name (much shorter) for these types.

This doesn't need to change anything else in Charon, the type abbreviations can still be inlined, it's just a matter of keeping the type alias also in scope for me to do something useful with it. Thanks!