crypto: cleanup, more testing

[alessio]

crypto.LedgerShowAddress() is a ledger specific function
and should takes acc address prefix instead of depending
on a sdk.Config singleton.

crypto/keys/mintkey:

• decryptPrivKey() return error instead of os.Exit()ing.
• encryptPrivKey() should panic instead of os.Exit() when
  bcrypt.GenerateFromPassword() call fails so that the
  caller is given a chance to recover().

crypto/keys:

• SignWithLedger() does not need to be method of
  baseKeybase. Method receiver is unused.
• ledgerInfo objecta may be pointers, type switch
  should handle both to avoid panics.
• keyringKeybase should not provide Update() at all.

More test cases are added to increase package crypto
and subpackages overall coverage.

---

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

---

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 #5735 into master will increase coverage by 0.28%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master    #5735      +/-   ##
==========================================
+ Coverage   36.22%   36.51%   +0.28%     
==========================================
  Files         335      336       +1     
  Lines       32722    32742      +20     
==========================================
+ Hits        11855    11956     +101     
+ Misses      19641    19557      -84     
- Partials     1226     1229       +3

Impacted Files	Coverage Δ
client/keys/show.go	83.9% <0%> (ø)	⬆️
crypto/keys/keybase.go	65.26% <0%> (ø)	⬆️
crypto/ledger_secp256k1.go	59.43% <100%> (+6.16%)	⬆️
crypto/keys/keyring.go	52.26% <100%> (+12.39%)	⬆️
crypto/keys/mintkey/mintkey.go	83.33% <33.33%> (+32.14%)	⬆️
crypto/keys/keybase_base.go	72.51% <60%> (+3.71%)	⬆️
crypto/keys/keyerror/errors.go	100% <0%> (ø)

[alexanderbez]

ACK