Support nested execution in the planner

[Arachnid]

If we do this via raw calls, it would work something like this:

• Add accessors on the planner .state and .commands that return placeholder Value objects representing the executor state and the commands used by a nested instance.
• Add a function on the planner .createSubplanner (or something). This function takes a FunctionCall, wraps it into a SubplanFunctionCall and puts it on the list of calls, then returns a new planner object initialised with the context of the parent.
• When planning, if we encounter a SubplanFunctionCall, recurse and plan that call before continuing. Use the same internal state for tracking slot liveness etc, as the same state array will be used throughout all the nested calls.
• When looking at function arguments and return values, if we encounter a StateValue, encode the special register ID for the state. If we encounter a CommandsValue and the current call is a SubplanFunctionCall, ABI-encode the the list of commands for that subplan and insert as a literal value into the state.
• If we encounter a CommandsValue in a call that's not a SubplanFunctionCall, throw an error. If we encounter a SubplanFunctionCall that doesn't have a CommandsValue, throw an error.