improve handling of u64 -> usize conversions at API border

[porcuquine]

Description

See discussion (#641 (comment)) leading up to and including the following summary:

• Use usize everywhere internally.
• Use u64 for API calls across the FFI border.
• Convert from u64 to usize in order to generate parameters required for both proving and verification.
• Fail with error if API client tries to replicate a too-large sector on a 32-bit system. i.e. bounds-check at conversion time, as early as possible.
• Note that because we only need to convert the number of 32-byte nodes to usize in order to verify, this will let us support up to 128GiB sectors on 32-bit systems (verification only). This is enough for now.

More detailed context here: #641 (comment)

Acceptance criteria

Risks + pitfalls

Where to begin