Fixed off-by-one error in the assembler's typeDig()

[algoidurovic]

Summary

This type checking bug was discovered while using the TEAL debugger. A correction to typedDig() + improved unit tests were added.

Test Plan

Added new unit test and corrected existing one.

[jannotti]

This looks good. Glad to see the extra test. I'll merge once CI compleetes.

[codecov-commenter]

Codecov Report

Merging #3056 (430552f) into master (3572e1f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3056      +/-   ##
==========================================
+ Coverage   43.65%   43.67%   +0.01%     
==========================================
  Files         391      391              
  Lines       86792    86792              
==========================================
+ Hits        37893    37906      +13     
+ Misses      42872    42861      -11     
+ Partials     6027     6025       -2     

Impacted Files	Coverage Δ
data/transactions/logic/assembler.go	79.26% <100.00%> (ø)
agreement/cryptoVerifier.go	75.73% <0.00%> (-2.21%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
ledger/acctupdates.go	66.53% <0.00%> (+0.09%)	⬆️
network/wsNetwork.go	63.14% <0.00%> (+0.18%)	⬆️
network/wsPeer.go	73.83% <0.00%> (+0.25%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️
txnsync/encodedgroupsmarshalers.go	100.00% <0.00%> (+1.12%)	⬆️
ledger/blockqueue.go	83.90% <0.00%> (+1.72%)	⬆️
ledger/roundlru.go	96.22% <0.00%> (+5.66%)	⬆️

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