Migrate slashing module to use new Simapp

[jgimeno]

Closes: #4875

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
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• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[alexanderbez]

LGTM. One thing I want to be very cautious of is that the helpers and aux methods and functions found in simapp/test_helpers.go don't become tech debt. After we've updated all the modules, we should go back through these and make sure every method and function makes sense, keeping it DRY and maintainable.

[codecov]

Codecov Report

Merging #5744 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5744      +/-   ##
==========================================
+ Coverage   36.79%   36.81%   +0.02%     
==========================================
  Files         336      336              
  Lines       32752    32751       -1     
==========================================
+ Hits        12051    12058       +7     
+ Misses      19473    19465       -8     
  Partials     1228     1228

Impacted Files	Coverage Δ
types/handler.go	77.77% <100%> (+77.77%)	⬆️