feat(trie): add empty variant support

[qdm12]

Changes

• Decode header using variant struct of bits + bit mask
• Add empty variant header
• Remove now unneeded encodeRoot (taken care by new variant) - no compatibility change at all
• (Side change) more readable decode test cases
• Do not allow header 00000000 when decoding, if v0 is used (blocked by versioning PR)
• Make sure encoding and decoding new lines are test covered

Note: this blocks addition of other node variants

Tests

go test -count 1  ./internal/trie/... ./lib/trie/...

Issues

#2418

Primary Reviewer

[codecov]

Codecov Report

Merging #2665 (67c79b5) into development (8fd2188) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #2665      +/-   ##
===============================================
+ Coverage        62.96%   62.97%   +0.01%     
===============================================
  Files              211      212       +1     
  Lines            27113    27148      +35     
===============================================
+ Hits             17071    17097      +26     
- Misses            8482     8489       +7     
- Partials          1560     1562       +2     

Flag	Coverage Δ
unit-tests	62.97% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
internal/trie/node/decode.go	95.55% <100.00%> (+0.10%)	⬆️
internal/trie/node/encode.go	93.75% <100.00%> (+0.56%)	⬆️
internal/trie/node/header.go	100.00% <100.00%> (ø)
internal/trie/node/variants.go	100.00% <100.00%> (ø)
lib/trie/trie.go	99.36% <100.00%> (-0.01%)	⬇️
lib/babe/verify.go	88.88% <0.00%> (-2.35%)	⬇️
dot/network/utils.go	58.33% <0.00%> (-1.93%)	⬇️
dot/sync/message.go	66.21% <0.00%> (-0.68%)	⬇️
dot/types/babe.go	34.72% <0.00%> (ø)
lib/runtime/wasmer/imports.go	55.43% <0.00%> (+0.17%)	⬆️
... and 2 more

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[dimartiro]

Closed in favour of #3295