Add Payload.Address() to allow migrations, etc. to reduce overhead

[fxamacker]

This PR adds Payload.Address() to extract and return payload address. This can be used (e.g. by migrations) to reduce overhead of extracting address from payload key.

Discussion 🧵 #5942 (comment)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.15385% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 55.80%. Comparing base (77de1a5) to head (53c94b2).
Report is 3 commits behind head on master.

Files	Patch %	Lines
ledger/trie_encoder.go	63.88%	8 Missing and 5 partials ⚠️
ledger/trie.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5948      +/-   ##
==========================================
+ Coverage   55.78%   55.80%   +0.01%     
==========================================
  Files        1128     1128              
  Lines       88977    88996      +19     
==========================================
+ Hits        49636    49662      +26     
+ Misses      34599    34591       -8     
- Partials     4742     4743       +1     

Flag	Coverage Δ
unittests	55.80% <71.15%> (+0.01%)	⬆️

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[turbolent]

Great refactor and improvement! 👏