Implement Get/Set operations on anchors

[lthibault]

API Design

• OCAP-MD's interfaces look like a good staring point
• See also: Sandstorm's Assignable interface (esp. asSetter and asGetter)

Implementation

• Values should be stored in a memdb table, separate from anchor.

Misc

• We want to support a security pattern for Proc that requires callers to provide the anchor to which the process should be assigned. The executor would assert that the anchor is a local one before starting the process.