merge crypto/keys/mintkey into crypto/

[alessio]

crypto/keys/mintkey provides only armoring functions.
It makes very little sense to keep it standalone and
under a name which does not really seem consistent with
the features it provides.

---

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
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

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

[codecov]

Codecov Report

Merging #5880 into master will not change coverage.
The diff coverage is 94.11%.

@@           Coverage Diff           @@
##           master    #5880   +/-   ##
=======================================
  Coverage   56.73%   56.73%           
=======================================
  Files         344      344           
  Lines       20294    20294           
=======================================
  Hits        11514    11514           
  Misses       7899     7899           
  Partials      881      881

Impacted Files	Coverage Δ
crypto/armor.go	87.5% <ø> (ø)
crypto/keyring/keyring.go	51.91% <100%> (ø)	⬆️
crypto/keyring/db_keybase.go	66.13% <90.9%> (ø)	⬆️

[alexanderbez]

ACK