refactor: StateStack perf, typecheck bugfix

[OttoHatt]

Performance improvements.
And, fixes a bug where pushing an invalid value into the StateStack fatally corrupts it (read comment).

local stack = StateStack.new("a", "string")
local clean = stack:PushState("b")
task.defer(stack.PushState, stack, 1)
task.defer(function()
	-- '1' was added to the internal stack.
	-- Cleaning pulls it back up, setting .Value =, throwing inline!
	clean()
	print("New value", stack:GetState())
end)