Refactor signature subpackets for better code and update FFI.

[ni4]

...and get closer to absence of anonymous unions and raw pointers.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.80338% with 87 lines in your changes missing coverage. Please review.

Project coverage is 84.65%. Comparing base (92adfd7) to head (ebcdb2e). Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/librepgp/stream-dump.cpp	86.30%	23 Missing :warning:
src/librepgp/stream-sig.cpp	88.83%	22 Missing :warning:
src/lib/sig_subpacket.cpp	94.03%	18 Missing :warning:
src/lib/sig_subpacket.hpp	91.97%	15 Missing :warning:
src/lib/rnp.cpp	89.36%	5 Missing :warning:
src/lib/fingerprint.cpp	75.00%	2 Missing :warning:
src/lib/pgp-key.cpp	92.85%	1 Missing :warning:
src/librepgp/stream-packet.cpp	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2281 +/- ## ========================================== + Coverage 84.27% 84.65% +0.38% ========================================== Files 114 116 +2 Lines 23324 23422 +98 ========================================== + Hits 19656 19829 +173 + Misses 3668 3593 -75 ```

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[ni4]

Merging with two approvals. Thanks all!