* split `decls.nim` into smaller modules - allows using parts of the ABI
without compiling all of `bearssl`
* deprecate functions with `Br` prefix - there are duplicate exports
both with and without `Br` for the same function and we use both in
consumers like `chronos` and `libp2p`
* fix several cases of incorrectly mapped types
* use `var` for certain arguments that can't be `nil`
* add script to regenerate ABI with `c2nim`
* consistently use `uint` for length (`int` was sometimes used)
The Split likely needs more cleanup work - this is a first cut to get
the idea in place.
In the new layout, `bearssl/abi/` contains "raw" nim mappings while
hand-written helpers are in `bearssl/`.
