Enter the new keyring interface

[alessio]

crypto/keyring:

Keybase interface gives way to its successor: Keyring. LegacyKeybase
interface is added in order to guarantee limited backward compatibility with
the old Keybase interface for the sole purpose of migrating keys across
the new keyring backends.

The package does no longer depends on the github.com/types.Config
singleton.

crypto/ subdirs reorganization: crypto/keys/hd was moved to crypto/hd,
which now groups together all HD Wallets related types and utilities.

SupportedAlgos and SupportedLedgerAlgos methods have been removed.
The keyring just fails when trying to perform an action with an unsupported
algorithm.

client/input:

• Removal of unnecessary GetCheckPassword, PrintPrefixed functions.
• GetConfirmation's signature changed to take in a io.Writer for better integration
  with cobra.Command types.

client/context:

• In-memory keyring is allocated in the context when --gen-only flag is passed
  in. GetFromFields does no longer silently allocate a keyring, it takes one as
  argument.

Co-authored with @jgimeno

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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
• 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)

[alexanderbez]

utACK