Introduce GasKey to the trie.

[robin-near]

See near/NEPs#611

This PR adds the GasKey trie key. In subsequent PRs we will introduce a new transaction type to accept gas key transactions, and to implement the gas key actions.

[pugachAG]

not sure if that is relevant, but SingleAccessKeyChanges uses find_exact_iter, maybe that is what we should use here as well

[robin-near]

Good eye! But here we do not want to use find_exact_iter because the prefix includes changes to the gas key itself (i.e. balance changes) as well as any of the nonces under the gas key. This is different from the access key case where there's only one raw key.

[codecov]

Codecov Report

Attention: Patch coverage is 0.53908% with 369 lines in your changes missing coverage. Please review.

Project coverage is 69.27%. Comparing base (25451c8) to head (d02409b).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
core/primitives/src/trie_key.rs	0.00%	77 Missing ⚠️
core/primitives/src/types.rs	0.00%	52 Missing ⚠️
tools/fork-network/src/storage_mutator.rs	0.00%	52 Missing ⚠️
tools/fork-network/src/cli.rs	0.00%	42 Missing ⚠️
core/store/src/genesis/state_applier.rs	0.00%	30 Missing ⚠️
tools/mirror/src/genesis.rs	0.00%	27 Missing ⚠️
core/primitives/src/views.rs	0.00%	26 Missing ⚠️
chain/chain/src/store/mod.rs	0.00%	19 Missing ⚠️
core/primitives/src/state_record.rs	0.00%	18 Missing ⚠️
core/store/src/utils/mod.rs	0.00%	17 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13687      +/-   ##
==========================================
- Coverage   69.41%   69.27%   -0.15%     
==========================================
  Files         956      956              
  Lines      184142   184513     +371     
  Branches   184142   184513     +371     
==========================================
- Hits       127830   127818      -12     
- Misses      51252    51632     +380     
- Partials     5060     5063       +3     

Flag	Coverage Δ
pytests	1.38% <0.00%> (-0.01%)	⬇️
pytests-nightly	1.45% <0.00%> (-0.01%)	⬇️
unittests	68.95% <0.53%> (-0.15%)	⬇️
unittests-nightly	68.78% <0.53%> (-0.15%)	⬇️

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