Add priv and public key bytes for HD wallet keys.

This commit introduces an HDPrivateKeyID and HDPublicKeyID field to the
params struct which are used by hierarchical deterministic extended keys
as defined by BIP0032.

In addition, a new function named HDPrivateKeyToPublicKeyID has been added
to allow the caller to get the associated public key ID given a private
key ID.

The tests have also been updated to maintain 100% test coverage.

ok @jrick
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.