[WIP] New ChannelId enum

[optout21]

Fixes #2408 . Introduces a new ChannelId enum, with two variants, for normal funding-tx-based and temporary ID, both wrapper around 32-byte data.

TODO:

• Finalize naming (common case is now called FundingTxBased)
• Determine how it affects APIs, uses, backward-compatibility
• How to preserve the variant at serialization? (both are stored as the 32 bytes)
• Review hints

[codecov-commenter]

Codecov Report

Patch coverage: 82.56% and project coverage change: +0.01% 🎉

Comparison is base (685f266) 90.33% compared to head (fcd7631) 90.35%.
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2456      +/-   ##
==========================================
+ Coverage   90.33%   90.35%   +0.01%     
==========================================
  Files         106      106              
  Lines       55732    55815      +83     
  Branches    55732    55815      +83     
==========================================
+ Hits        50347    50433      +86     
+ Misses       5385     5382       -3     

Files Changed	Coverage Δ
lightning/src/routing/gossip.rs	89.67% <0.00%> (-0.07%)	⬇️
lightning/src/ln/peer_handler.rs	61.40% <13.79%> (-0.21%)	⬇️
lightning/src/routing/router.rs	93.57% <50.00%> (-0.06%)	⬇️
lightning/src/util/macro_logger.rs	89.58% <50.00%> (ø)
lightning/src/events/mod.rs	43.40% <76.47%> (+1.30%)	⬆️
lightning/src/ln/msgs.rs	85.51% <81.66%> (+0.54%)	⬆️
lightning/src/ln/channelmanager.rs	85.73% <88.37%> (+0.01%)	⬆️
lightning/src/ln/channel.rs	89.55% <95.74%> (+0.09%)	⬆️
lightning-invoice/src/utils.rs	97.69% <100.00%> (ø)
lightning/src/chain/channelmonitor.rs	94.82% <100.00%> (ø)
... and 5 more

... and 5 files with indirect coverage changes

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[optout21]

Not intended for merging, see comment in #2408 #2408 (comment)
Closing