Update to latest ZFEC API in the Botan branch

The C API in Botan for ZFEC changed since we wrote the FFI code in tahoe-chk.

This updates the Botan flake input to the latest revision of the branch, updates the FFI code to use the new Botan C API, and makes some minor nearby cleanups (simplifying expressions through better choice of composition operators and function argument order, removing unused functions, adding docs, etc).