feat: prepare for testing fraudulant blocks by allowing for custom hashers and the ability to add leaves without validation

[evan-forbes]

Overview

Currently, the hasher is not swappable, this is only problematic when trying to create fraudulent block test cases, since the standard hasher prevents such behavior.

Checklist

• [x] New and updated code has appropriate documentation
• [x] New and updated code has new and/or updated testing
• [x] Required CI checks are passing
• [x] Visual proof for any user facing features like CLI or documentation updates
• [ ] Linked issues closed with keywords

[codecov[bot]]

Codecov Report

Merging #211 (6dc07a6) into master (cdc88e8) will decrease coverage by 0.20%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   94.03%   93.83%   -0.20%     
==========================================
  Files           5        5              
  Lines         570      584      +14     
==========================================
+ Hits          536      548      +12     
- Misses         19       20       +1     
- Partials       15       16       +1     

Impacted Files	Coverage Δ
proof.go	88.51% <ø> (ø)
nmt.go	96.42% <87.50%> (-0.72%)	:arrow_down:
hasher.go	97.56% <100.00%> (ø)

[evan-forbes]

However, it will require additional time to thoroughly identify the specific issues that users may encounter without proper implementation of the Hasher interface. We may want to look into this later. As we are currently using our own implementation that should be fine.

pretty much the only reason I can think of to do this is to purposefully mess things up lolol

left a note in the godoc for the hasher interface to say just that