[AST] Introduce ActiveRequest

This wrapper is used to type erase a reference to
a request on the stack, and can be converted into
an AnyRequest when persistent storage is required.

Switch the evaluator over to using ActiveRequest
for its stack of active requests, meaning it now
only needs to heap allocate requests when they
enter the cache, or if it's building the (disabled
by default) dependency graph.

Author: hamishknight