statetrie: trie struct, node structs, and add operation

[bbroder-uji]

This PR adds the trie struct and nodes, plus the add operation to add key/value pairs to the trie.

[bbroder-uji]

this PR requires #5759 first

[codecov]

Codecov Report

Attention: 116 lines in your changes are missing coverage. Please review.

Comparison is base (e7aa0d4) 55.95% compared to head (43d40d1) 55.95%.

Files	Patch %	Lines
crypto/statetrie/branch.go	62.16%	24 Missing and 4 partials ⚠️
crypto/statetrie/ext.go	71.27%	17 Missing and 10 partials ⚠️
crypto/statetrie/stats.go	0.00%	19 Missing ⚠️
crypto/statetrie/nodes.go	0.00%	12 Missing ⚠️
crypto/statetrie/backing.go	54.54%	10 Missing ⚠️
crypto/statetrie/leaf.go	85.91%	10 Missing ⚠️
crypto/statetrie/trie.go	65.51%	5 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5767      +/-   ##
==========================================
- Coverage   55.95%   55.95%   -0.01%     
==========================================
  Files         477      484       +7     
  Lines       67436    67757     +321     
==========================================
+ Hits        37736    37911     +175     
- Misses      27149    27274     +125     
- Partials     2551     2572      +21     

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