Replace checkpoint system

The previous system needed to store of the scope and full AST to build a Resumable object, which means that if persisted, updating the resumable script will have no effect.
The new system instead uses an anchor token and does not require any information besides the anchor name.

ast/Anchor.lua

@@ -0,0 +1,35 @@
+local ast = require("ast")
+
+local resume_manager
+
+local Anchor
+Anchor = ast.abstract.Node {
+	type = "anchor",
+
+	name = nil,
+
+	init = function(self, name)
+		self.name = name
+		self._list_anchors_cache = { [name] = true }
+	end,
+
+	_hash = function(self)
+		return ("anchor<%q>"):format(self.name)
+	end,
+
+	_format = function(self, ...)
+		return "#"..self.name
+	end,
+
+	_eval = function(self, state)
+		if self:contains_resume_target(state) then
+			resume_manager:set_reached(state)
+		end
+		return Anchor:new(self.name)
+	end
+}
+
+package.loaded[...] = Anchor
+resume_manager = require("state.resume_manager")
+
+return Anchor