Principal namespaces

[emilypi]

This PR modifies the namespace contract ns to allow for autonomously-generated namespaces using principal account protocols. The schema is as follows:

1. Users may create a principal namespace name using the new create-principal-namespace function added to the contract in this PR for the admin guard they might care to use to govern their namespace.
2. Autonomously generated namespace names are of the format n_ + (160 bit truncations of Blake2b_256 hash of guard).
3. When issuing define-namespaace and namespace in a Pact script, if the namespace name is the same as the created name (checked against the admin guard), then usage will succeed and otherwise fail.