Make encode_vec() infallible

[hu55a1n1]

Closes: #1064

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #1093 (5010ee2) into master (904862c) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #1093     +/-   ##
========================================
- Coverage    64.6%   64.6%   -0.1%     
========================================
  Files         239     239             
  Lines       20695   20691      -4     
========================================
- Hits        13387   13381      -6     
- Misses       7308    7310      +2     

Impacted Files	Coverage Δ
proto/src/lib.rs	100.0% <100.0%> (ø)
proto/tests/unit.rs	94.8% <100.0%> (ø)
tendermint/src/block/header.rs	99.2% <100.0%> (-0.1%)	⬇️
tendermint/src/public_key.rs	75.1% <100.0%> (ø)
tendermint/src/validator.rs	73.6% <100.0%> (ø)
tendermint/src/vote/sign_vote.rs	96.5% <100.0%> (ø)
tendermint/src/block/commit_sig.rs	78.4% <0.0%> (-2.0%)	⬇️
light-client-verifier/src/types.rs	37.9% <0.0%> (-0.6%)	⬇️
abci/src/server.rs	9.4% <0.0%> (+0.1%)	⬆️
testgen/src/header.rs	84.4% <0.0%> (+0.5%)	⬆️

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[thanethomson]

Just one minor recommendation regarding the changelog entry - I'll make the change.

Thanks @hu55a1n1!