go/common/encoding/bech32: Replace to-be-removed dependency

[kostko]

The github.com/tendermint/tendermint/libs/bech32 module has been removed in
later Tendermint Core versions. As it only contained a small helper, we just
copy it over.

[codecov]

Codecov Report

Merging #3030 into master will increase coverage by 0.07%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #3030      +/-   ##
==========================================
+ Coverage   68.44%   68.51%   +0.07%     
==========================================
  Files         367      367              
  Lines       36126    36135       +9     
==========================================
+ Hits        24725    24758      +33     
+ Misses       8244     8186      -58     
- Partials     3157     3191      +34     

Impacted Files	Coverage Δ
go/common/encoding/bech32/bech32.go	69.23% <63.63%> (-30.77%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/worker/compute/executor/committee/fault.go	68.96% <0.00%> (-8.05%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/storage/metrics.go	78.26% <0.00%> (-6.53%)	⬇️
go/storage/mkvs/lookup.go	73.46% <0.00%> (-4.09%)	⬇️
go/consensus/tendermint/abci/state.go	67.62% <0.00%> (-3.60%)	⬇️
go/storage/mkvs/tree.go	89.28% <0.00%> (-3.58%)	⬇️
go/storage/api/root_cache.go	70.11% <0.00%> (-2.30%)	⬇️
go/storage/mkvs/cache.go	81.27% <0.00%> (-1.71%)	⬇️
... and 25 more

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