Increase coverage in AVM code. Bug fix in account fields of tx_field

[jannotti]

Intended to just increase coverage in AVM, but doing so found a
bug. (Go figure.) tx_field should only allow actual addresses for
address fields, rather than low integers as references from the
Account array.

Test Plan

Lots more tests that increase coverage, especially for tx_field.

[codecov-commenter]

Codecov Report

Merging #2840 (01a168e) into master (e32b2c2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2840   +/-   ##
=======================================
  Coverage   47.28%   47.29%           
=======================================
  Files         351      351           
  Lines       56381    56387    +6     
=======================================
+ Hits        26659    26666    +7     
- Misses      26730    26731    +1     
+ Partials     2992     2990    -2     

Impacted Files	Coverage Δ
data/transactions/logic/eval.go	90.31% <100.00%> (+1.12%)	⬆️
ledger/blockqueue.go	82.18% <0.00%> (-2.88%)	⬇️
agreement/cryptoVerifier.go	75.73% <0.00%> (-2.21%)	⬇️
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
network/requestTracker.go	70.25% <0.00%> (-0.87%)	⬇️
catchup/service.go	69.35% <0.00%> (-0.78%)	⬇️
ledger/acctupdates.go	62.63% <0.00%> (+0.08%)	⬆️
... and 2 more

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