Define "implements" checks using internal slots

- Update the steps for instantiating a platform object, which everything
  theoretically calls into, and which sets the primary interface into a new
  internal slot [[PrimaryInterface]].
- Define the "implements" algorithm as checking the [[PrimaryInterface]]
  internal slot.

The algorithm here is roughly in correspondence with the V8 implementation,
where operation functions are allocated with a reference to a
FunctionTemplate object (which is in corresondence with WebIDL interfaces).
When checking the receiver of a method that came from WebIDL, the original
prototype chain is traversed by looking at the FunctionTemplate's parent.
The same FunctionTemplate is used in multiple JavaScript realms.

Fixes #97.