Fix module interface registration

[aaronc]

Previously InterfaceModule had been added as a separate module interface from AppModule.... It turns out that almost all modules need to register some interfaces (usually for sdk.Msg at a minimum. So separating this doesn't make sense. Also x/slashing and x/staking were missing interface registrations.

This PR:

• merges the InterfaceModule functionality into AppModuleBasic, renaming RegisterInterfaceTypes -> RegisterInterfaces
• adds missing registrations for x/slashing and x/staking

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #6834 into master will increase coverage by 3.27%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #6834      +/-   ##
==========================================
+ Coverage   61.40%   64.68%   +3.27%     
==========================================
  Files         414      379      -35     
  Lines       25084    23651    -1433     
==========================================
- Hits        15404    15298     -106     
+ Misses       8398     7218    -1180     
+ Partials     1282     1135     -147