Use Hash160 for strongly typed arguments

[GeneFerneau]

Use bitcoin::hashes::hash160::Hash in place of [u8; 20] for strongly typed arguments across the code base.

[codecov-commenter]

Codecov Report

Merging #33 (1f93217) into master (b67dab5) will decrease coverage by 0.03%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   80.89%   80.86%   -0.04%     
==========================================
  Files           8        8              
  Lines        2586     2587       +1     
==========================================
  Hits         2092     2092              
- Misses        494      495       +1     

Impacted Files	Coverage Δ
src/wallet_sync.rs	78.45% <0.00%> (ø)
src/contracts.rs	92.01% <100.00%> (-0.21%)	⬇️
src/maker_protocol.rs	80.77% <100.00%> (ø)
src/messages.rs	86.90% <100.00%> (ø)
src/taker_protocol.rs	95.07% <100.00%> (ø)

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[mojoX911]

Strong concept with review ACK.

We should try to move away from raw data wherever possible.