deps: Switch to openrlp crate

[sterliakov]

Only one quirk: openrlp serializes Vec<u8> as Bytes. That's arguably more consistent, and we only had one field to adjust

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.51%. Comparing base (f3e5c91) to head (5699ade).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/rlp.rs	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   93.47%   93.51%   +0.03%     
==========================================
  Files           7        7              
  Lines         644      632      -12     
==========================================
- Hits          602      591      -11     
+ Misses         42       41       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.