support Pubkey::to_bytes as constexpr

[ripatel-fd]

Problem

Cannot do compile-time tricks with static const pubkeys

Summary of Changes

Adds support for constexprs

Fixes #

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (e682fec) to head (6109c3e).

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #220     +/-   ##
=========================================
- Coverage    81.9%    81.8%   -0.1%     
=========================================
  Files         837      837             
  Lines      226539   226539             
=========================================
- Hits       185548   185534     -14     
- Misses      40991    41005     +14     

[joncinque]

Thanks for the contribution! This will definitely help, and for anyone else who ends up reading this, we can't do this with a From trait because const traits aren't stable in Rust yet.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.