turn 'phoenix-core' into a 'phoenix' workspace

[moCello]

Summary

With the decision to move the transfer circuits out of 'rusk' and into the phoenix implementation library #170, we have decided to create a new workspace called 'phoenix' with the following structure:

phoenix
  |- core
  |- circuits
  |- docs

The first step and scope of this issue is to add the workspace and its members. This will also include adding the existing elgamal encryption into the 'circuits' member.

The next step is adding the actual transfer circuits and gadgets as part of #170