feat(resharding) - Make shard ids non-contiguous - part 2

[wacban]

More of the same. I added a rust feature to quickly switch between the old and new shard ids to speed up work.

No interesting changes here, just syntax everywhere. I hope.

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.75%. Comparing base (2b022a6) to head (450ac7b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/primitives/src/shard_layout.rs	98.00%	0 Missing and 1 partial ⚠️
core/primitives/src/trie_key.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12209   +/-   ##
=======================================
  Coverage   71.74%   71.75%           
=======================================
  Files         825      825           
  Lines      165819   165865   +46     
  Branches   165819   165865   +46     
=======================================
+ Hits       118973   119018   +45     
  Misses      41650    41650           
- Partials     5196     5197    +1     

Flag	Coverage Δ
backward-compatibility	0.17% <0.00%> (-0.01%)	⬇️
db-migration	0.17% <0.00%> (-0.01%)	⬇️
genesis-check	1.26% <8.19%> (+<0.01%)	⬆️
integration-tests	38.93% <55.76%> (+0.02%)	⬆️
linux	71.45% <98.07%> (+0.01%)	⬆️
linux-nightly	71.32% <98.07%> (-0.01%)	⬇️
macos	54.34% <96.15%> (+0.07%)	⬆️
pytests	1.58% <8.19%> (+<0.01%)	⬆️
sanity-checks	1.38% <0.00%> (-0.01%)	⬇️
unittests	65.52% <97.11%> (+<0.01%)	⬆️
upgradability	0.21% <0.00%> (-0.01%)	⬇️

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