Use `hs-bindgen` to generate bindings

[jorisdral]

https://github.com/well-typed/hs-bindgen is a nifty new tool that can automate the process of writing Haskell bindings for C headers. Seeing as botan-bindings consists of mostly foreign import declarations, type definitions and utility pattern synonyms -- things that hs-bindgen generates automatically -- we can probably just generate most of the code for botan-bindings using hs-bindgen

Downside:

• Reliance on an external tool introduces a point of failure
• hs-bindgen is customisable in some parts, but not everywhere, so some fiddling with CPP will still be needed

Benefits:

• Easy to create new bindings when a new version of the C++ library is released
• The generated code has a consistent approach and style
• Reduced maintenance cost

[jorisdral]

I've added a few sub-issues that I think we can tackle as part of this issue. If we're changing botan-bindings, we can make these smaller changes too while we're at it.