IncrementalExecutor is an implementation detail of the Interpreter. In
order to test extended features properly, we must be able to setup and
tear down the executor manually.
RuntimeInterfaceBuilder wires up JITed expressions with the hardcoded
Interpreter runtime. It's used only for value printing right now, but it
is not limited to that. The default implementation focuses on an
evaluation process where the Interpreter has direct access to the memory
of JITed expressions (in-process execution or shared memory).
We need a different approach to support out-of-process evaluation or
variations of the runtime. It seems reasonable to expose a minimal
interface for it. The new RuntimeInterfaceBuilder is an abstract base
class in the public header. For that, the TypeVisitor had to become a
component (instead of inheriting from it). FindRuntimeInterface() was
adjusted to return an instance of the RuntimeInterfaceBuilder and it can
be overridden from derived classes.
