Broke up serializers.rs into its own folder

[greg-szabo]

This is the next step in refactoring serializers (#247 ). It breaks up the serializers.rs file into its own directory. No other changes were done. (The use clauses were updated accordingly in the new files.)

The "custom" serializers are the ones we'd like to get rid of. In the next step, when CommitSig is introduced, some of these can already go away.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGES.md

[codecov-io]

Codecov Report

Merging #272 into master will decrease coverage by 0.0%.
The diff coverage is 20.0%.

@@           Coverage Diff            @@
##           master    #272     +/-   ##
========================================
- Coverage    27.4%   27.4%   -0.1%     
========================================
  Files          99     101      +2     
  Lines        3694    3696      +2     
  Branches     1169    1169             
========================================
  Hits         1014    1014             
- Misses       1870    1872      +2     
  Partials      810     810             

Impacted Files	Coverage Δ
tendermint/src/serializers/custom.rs	13.0% <13.6%> (ø)
tendermint/src/serializers/from_str.rs	22.2% <28.5%> (ø)
tendermint/src/serializers/bytes.rs	31.5% <31.5%> (ø)

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[liamsi]

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