Investigate where we enforce the call stack limit

[SamWilsn]

It looks like we might enforce it in multiple places:

execution-specs/src/ethereum/cancun/vm/instructions/system.py

Lines 292 to 295 in 6e65228

	if evm.message.depth + Uint(1) > STACK_DEPTH_LIMIT:
	evm.gas_left += gas
	push(evm.stack, U256(0))
	return

And later:

execution-specs/src/ethereum/cancun/vm/interpreter.py

Lines 227 to 228 in 6e65228

	if message.depth > STACK_DEPTH_LIMIT:
	raise StackDepthLimitError("Stack depth limit reached")

One of these (interpreter.py) might be better suited as an assertion to catch bugs.