Export io::ErrorKind in bindings

[TheBlueMatt]

The bindings have exported io::Error as, basically, io::ErrorKind, for quite some time, so there's little reason to not just export io::ErrorKind as well.

[codecov-commenter]

Codecov Report

Base: 90.72% // Head: 90.71% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (2c69e05) compared to base (2e343e7).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1807      +/-   ##
==========================================
- Coverage   90.72%   90.71%   -0.01%     
==========================================
  Files          87       87              
  Lines       47364    47364              
  Branches    47364    47364              
==========================================
- Hits        42970    42966       -4     
- Misses       4394     4398       +4     

Impacted Files	Coverage Δ
lightning/src/ln/msgs.rs	86.24% <0.00%> (ø)
lightning/src/ln/peer_channel_encryptor.rs	93.38% <0.00%> (-0.25%)	⬇️
lightning/src/ln/functional_tests.rs	96.91% <0.00%> (-0.06%)	⬇️
lightning/src/chain/channelmonitor.rs	90.65% <0.00%> (-0.06%)	⬇️
lightning/src/ln/channelmanager.rs	85.18% <0.00%> (+0.02%)	⬆️
lightning/src/ln/monitor_tests.rs	99.55% <0.00%> (+0.11%)	⬆️

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