rustfmt: Run on util/* (1/2)

[tnull]

The diff is a bit larger for this one, but AFAICT the changes look mostly reasonable (besides the oddity commented below).

FWIW, I had a look at currently open inflight PRs and the conflicts should be minimal if I'm not overlooking something.

This is PR 1/2, follow-up: #3323.

[codecov]

Codecov Report

Attention: Patch coverage is 89.57055% with 34 lines in your changes missing coverage. Please review.

Project coverage is 89.64%. Comparing base (66fb520) to head (d68a484).

Files with missing lines	Patch %	Lines
lightning/src/util/persist.rs	89.41%	17 Missing and 3 partials ⚠️
lightning/src/util/macro_logger.rs	87.87%	0 Missing and 4 partials ⚠️
lightning/src/util/message_signing.rs	50.00%	4 Missing ⚠️
lightning/src/util/errors.rs	72.72%	3 Missing ⚠️
lightning/src/util/indexed_map.rs	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3324      +/-   ##
==========================================
+ Coverage   89.59%   89.64%   +0.04%     
==========================================
  Files         126      126              
  Lines      102676   102817     +141     
  Branches   102676   102817     +141     
==========================================
+ Hits        91995    92171     +176     
+ Misses       7947     7923      -24     
+ Partials     2734     2723      -11     

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[TheBlueMatt]

Everything looks good but the last commit, which starts to get into our functional tests which need to be cleaned up to be more readable, allowing them to also be more condensed.

[tnull]

Rebased after #3327 landed and addressed outstanding comments.