Functionize additional macro code in `channelmanager.rs`

[TheBlueMatt]

On my machine this decreases the build step of trivial-change lightning-crate test re-builds by around 10%

[ldk-reviews-bot]

👋 Thanks for assigning @joostjager as a reviewer! I'll wait for their review and will help manage the review process. Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 91.66667% with 13 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.33%. Comparing base (bda19d6) to head (7cea2e9). :warning: Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	91.66%	10 Missing and 3 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4224      +/-   ##
==========================================
- Coverage   89.34%   89.33%   -0.01%     
==========================================
  Files         180      180              
  Lines      138341   138469     +128     
  Branches   138341   138469     +128     
==========================================
+ Hits       123596   123706     +110     
- Misses      12123    12137      +14     
- Partials     2622     2626       +4     

Flag	Coverage Δ
fuzzing	35.98% <60.25%> (+0.06%)	:arrow_up:
tests	88.70% <91.66%> (-0.01%)	:arrow_down:

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[ldk-reviews-bot]

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[TheBlueMatt]

Squashed and rebased, fixing the commit message.