AVM: Rework around opcode fields for more flexible costs

[jannotti]

This PR makes is possible for opcodes to have different costs depending on their immediate argument. It also opens the door to opcodes that have different costs based on their run-time arguments (on stack).

There was significant rework around how information about opcode fields (immediates) is stored. This rework:

1. Makes doc generation more automatic
2. Eliminates the need for custom opcode disassemblers
3. Eliminates one hashmap lookup per evaluation of opcodes with immediate fields.
4. Places more documentation "inline" with definitions - immediates for opcodes, and docs for fieldspecs.

[codecov-commenter]

Codecov Report

Merging #3832 (8256400) into master (bc19540) will decrease coverage by 0.01%.
The diff coverage is 72.75%.

@@            Coverage Diff             @@
##           master    #3832      +/-   ##
==========================================
- Coverage   49.98%   49.96%   -0.02%     
==========================================
  Files         392      392              
  Lines       68501    68386     -115     
==========================================
- Hits        34239    34169      -70     
+ Misses      30501    30475      -26     
+ Partials     3761     3742      -19     

Impacted Files	Coverage Δ
data/transactions/logic/fields_string.go	40.00% <ø> (ø)
data/transactions/logic/doc.go	45.45% <20.00%> (-10.55%)	⬇️
data/transactions/logic/fields.go	46.80% <59.04%> (-2.43%)	⬇️
data/transactions/logic/eval.go	89.60% <83.78%> (-0.08%)	⬇️
data/transactions/logic/assembler.go	81.33% <83.96%> (+1.37%)	⬆️
data/transactions/logic/opcodes.go	97.05% <90.47%> (-2.95%)	⬇️
agreement/cryptoVerifier.go	68.08% <0.00%> (-2.13%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
data/transactions/verify/txn.go	44.15% <0.00%> (-0.87%)	⬇️
network/wsNetwork.go	62.79% <0.00%> (-0.20%)	⬇️
... and 7 more

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[michaeldiamant]

@jannotti Thanks for finding a way to support variable field costs while also simplifying opcode maintenance. ☕