Revert fillBytes method to bigIntToBytes for lower golang version

[ahangsu]

Summary

We are probably not going to higher golang version (1.15+) at this point, while fillBytes from big.Int is introduced in 1.15.
Changing all the fillBytes in ABI to self-made bigIntToBytes in favor of lower go version.

[jannotti]

I might have preferred a fill-in method with the exact same signature and semantics as FillBytes, so the change would be even smaller, but I'mm totally fine with this.

Let's merge after checks pass.

[jannotti]

I might have preferred a fill-in method with the exact same signature and semantics as FillBytes, so the change would be even smaller, but I'mm totally fine with this.

Let's merge after checks pass.

For what it's worth, emulating FillBytes exactly would be annoying, as it is willing to take a buffer of any size that's big enough. Our use case only needs to work on exactly the right size buffer.

[jasonpaulos]

Looks correct. I agree with the earlier comments about copy and I left another, but they are minor

[ahangsu]

I tried to emulate a method like FillBytes before, but we would need to introduce new type alias for big.Int (in favor of defining new methods), plus a bunch of type casting to use new method, and I am reluctant to create a larger delta.

The current fix should be sufficient for the purpose of casting big.Int to specific length of byte slice, please let me know your thoughts.

[codecov-commenter]

Codecov Report

Merging #3498 (17b964e) into master (2edd3de) will decrease coverage by 0.02%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master    #3498      +/-   ##
==========================================
- Coverage   47.67%   47.64%   -0.03%     
==========================================
  Files         370      370              
  Lines       59832    59841       +9     
==========================================
- Hits        28522    28514       -8     
- Misses      28002    28011       +9     
- Partials     3308     3316       +8     

Impacted Files	Coverage Δ
data/abi/abi_encode.go	64.08% <45.45%> (-0.82%)	⬇️
cmd/algoh/blockWatcher.go	77.77% <0.00%> (-3.18%)	⬇️
ledger/blockqueue.go	82.18% <0.00%> (-2.88%)	⬇️
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
catchup/service.go	69.40% <0.00%> (-0.50%)	⬇️
network/wsNetwork.go	63.29% <0.00%> (+0.29%)	⬆️
network/wsPeer.go	68.88% <0.00%> (+0.55%)	⬆️

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