Consolidation of crypto functions

Depends on #174.

This pull request removes various hex functions from `convert`, bringing us one step closer to removing it all together; and moves `HmacSHA512` from `HDWallet` to `crypto`.

For `HmacSHA512`, asserts have been used to ensure that any transition does not cause any issues.
Tests have also been added to ensure the output of `HmacSHA512` is as expected.

Will rebase on master when/if #174 is merged.