Add matching serializer for app_hash field of block::Header struct

[romac]

Closes: #187

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

[liamsi]

👍

[liamsi]

Very small change with relatively a lot of test code. All our PRs should be a bit more like @romac's :-) 👍

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@50cd585). Click here to learn what that means.
The diff coverage is 56.25%.

@@            Coverage Diff            @@
##             master     #188   +/-   ##
=========================================
  Coverage          ?   39.72%           
=========================================
  Files             ?       98           
  Lines             ?     3522           
  Branches          ?      559           
=========================================
  Hits              ?     1399           
  Misses            ?     1722           
  Partials          ?      401

Impacted Files	Coverage Δ
tendermint/src/block/header.rs	20.83% <100%> (ø)
tendermint/src/test.rs	41.66% <41.66%> (ø)

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