AVM: Provide access to some more block header values

[jannotti]

Because of block incentives, there are good reasons to expose the FeeSink and the ProposerPayout from the block header to AVM using the block opcode. While doing that, we can consider a few more things that might be useful.

Currently this PR exposes these.

// BlkBranch is the hash of the previous block
BlkBranch

// BlkFeeSink is the fee sink for the given round
BlkFeeSink

// BlkProtocol is the ConsensusVersion of the block.
BlkProtocol

// BlkTxnCounter is the number of the next transaction after the block
BlkTxnCounter

// BlkProposerPayout is the actual amount moved from feesink to proposer
BlkProposerPayout

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.27%. Comparing base (4990077) to head (4fdcff6).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6107      +/-   ##
==========================================
- Coverage   56.27%   56.27%   -0.01%     
==========================================
  Files         490      490              
  Lines       69710    69720      +10     
==========================================
+ Hits        39230    39234       +4     
- Misses      27817    27819       +2     
- Partials     2663     2667       +4     

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[jasonpaulos]

Looks good but I did want to call attention to the change in values for the other v11 block fields