The purpose is to make it easier to add new scripting functionality without adding new protobuf interfaces. A method call is just like a function call except:

1) It is marked as a method call in the RPC request
2) It must contain an "id" argument
3) The value of the "id" argument is a globally unique identifier for the receiver

This commit adds support for a class creating a variables frame to mark one of its variables as a "primary key". Primary keys are added to the global index, and can be targeted by method calls.

Varible references have been generalized not to depend on scopes, so that a variable may hold a reference to its own primary key.

A class may support methods by:
1) Conforming to iTermObject, which is necessary to be the "owner" of an iTermVariables object.
2) Returning built-in functions from -objectMethodRegistry.

Those built-in functions should be named as fooWithCompletion: followed by zero or more other arguments. Reflection is used to type check inputs, although it's unfortunately ineffective as it only recognizes NSObject. Better type checking is still needed.