chore(trie): refactor existing header encoding

[qdm12]

Changes

TLDR: Match more closely the polkadot spec documentation in the code, to allow further changes for new node variants. This doesn't add new node variants yet, this is coming in the next PR.

• Declare node variants as bits+mask instead of just bits (or the more obscure enum integer we had)
• encodeHeader encodes the first header byte + extra partial key length bytes instead of the convoluted/not-modular encodeHeader + encodeKeyLength
• decodeHeader function to get variant + partial key length, instead of convoluted in-line header code in Decode + further header decoding in decodeLeaf and decodeBranch
• Get node variant header partial key length mask dynamically from the node variant header bit mask (not a constant anymore)
• Clarify codec documentation in a README.md document
• Remove single byte sync pool (slower than stack byte slice of length 1)
• ⚠️ Fix decodeKey maximum check for last byte
• ⚠️ Fix decodeKey maximum check to accept maximum value 65535
• ⚠️ allow to encode partial key of size 65535
• Panic for programming errors cases
• Revise some of the error wrapping
• Use uint16 for partialKeyLength

Tests

go test lib/trie/... internal/trie/...

Issues

• Implement storage trie version 1 #2418

Primary Reviewer

@EclesioMeloJunior

[codecov]

Codecov Report

Merging #2530 (374a29b) into development (5dc567e) will decrease coverage by 0.04%.
The diff coverage is 94.61%.

@@               Coverage Diff               @@
##           development    #2530      +/-   ##
===============================================
- Coverage        61.98%   61.94%   -0.05%     
===============================================
  Files              215      215              
  Lines            28450    28485      +35     
===============================================
+ Hits             17636    17644       +8     
- Misses            9064     9087      +23     
- Partials          1750     1754       +4     

Flag	Coverage Δ
unit-tests	61.94% <94.61%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/trie/node/encode.go	93.33% <ø> (ø)
internal/trie/node/node.go	100.00% <ø> (ø)
internal/trie/node/decode.go	91.11% <78.12%> (-4.81%)	⬇️
internal/trie/node/header.go	100.00% <100.00%> (ø)
internal/trie/node/key.go	100.00% <100.00%> (ø)
dot/network/inbound.go	92.98% <0.00%> (-7.02%)	⬇️
dot/rpc/modules/grandpa.go	83.72% <0.00%> (-5.44%)	⬇️
dot/network/block_announce.go	58.40% <0.00%> (-4.81%)	⬇️
dot/network/connmgr.go	89.04% <0.00%> (-1.37%)	⬇️
lib/blocktree/blocktree.go	53.62% <0.00%> (-1.09%)	⬇️
... and 7 more

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[github-actions]

🎉 This PR is included in version 0.7.0 🎉

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