Custom bindings to libclang

[edsko]

There are two existing bindings on hackage:

• https://hackage.haskell.org/package/LibClang
• https://hackage.haskell.org/package/clang-pure

Neither of which seems particularly good. The former is last updated in 2017; the latter is only 1500 lines of Haskell, but all undocumented, it implements some crazy reference counting, for reasons unclear, and doesn't have many of the features we need.

The goal here is not to we write our own general purpose bindings, but instead implement some sane bindings for our own use. Unlike what clang-pure does, I think we should not provide any kind of cursors, or try to share memory with libclang, but instead traverse the AST using the libclang interface, construct a pure Haskell value it, managed on the Haskell heap, and then be done with libclang from that point on.