Add feature to export key revocation.

[ni4]

This PR is part of work on key revocation(s) (issue #1005), and fixes #1007
It implements low-level functionality to generate key revocation certificate, FFI function to export such certificate and rnpkeys CLI command --export-rev and options --rev-type, --rev-reason to work with this.

[codecov]

Codecov Report

Merging #1066 into master will increase coverage by 0.1%.
The diff coverage is 83.24%.

@@            Coverage Diff            @@
##           master    #1066     +/-   ##
=========================================
+ Coverage   82.96%   83.06%   +0.1%     
=========================================
  Files          76       76             
  Lines       28482    28748    +266     
=========================================
+ Hits        23629    23880    +251     
- Misses       4853     4868     +15

Impacted Files	Coverage Δ
src/lib/pgp-key.cpp	80.22% <ø> (ø)	⬆️
src/tests/ffi.cpp	98.78% <ø> (+0.02%)	⬆️
src/tests/exportkey.cpp	100% <100%> (ø)	⬆️
src/tests/generatekey.cpp	96.78% <100%> (ø)	⬆️
src/tests/issues/1030.cpp	100% <100%> (ø)	⬆️
src/librepgp/stream-packet.cpp	77.06% <100%> (ø)	⬆️
src/tests/streams.cpp	98.82% <100%> (ø)	⬆️
src/librekey/key_store_pgp.cpp	68.42% <100%> (+0.15%)	⬆️
src/librepgp/stream-key.cpp	66.54% <52.08%> (-0.92%)	⬇️
src/rnpkeys/rnpkeys.cpp	65.71% <90.47%> (+3.34%)	⬆️
... and 10 more

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[rrrooommmaaa]

I have read thoroughly the unit test. Why don't you use rnp_output_to_null?

[ni4]

@dewyatt @ronaldtse Could you please review this? I wasn't able to satisfy codecov however I tried as hard as possible.

[dewyatt]

LGTM

[ronaldtse]

LGTM!