Implement the Key Module of the Morphir SDK

[michelchan]

Is your feature request related to a problem? Please describe.
Morphir's SDK seeks to have parity with much of elm/core and should have full parity with elm/core Key.

Describe the solution you'd like
Implement the functions in elm/core Key and add tests. Over time we will add more functionality here, but this is a good start.

Describe alternatives you've considered
N/A

Additional context
N/A