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.

state/resume_manager.lua

@@ -0,0 +1,44 @@
+local class = require("class")
+
+local ast = require("ast")
+local Nil, Identifier, Anchor
+
+-- stack of resumable contexts
+local resume_anchor_identifier, resume_anchor_symbol
+
+local resume_manager = class {
+	init = false,
+
+	-- push a new resume context: all run code between this and the next push will try to resume to anchor
+	push = function(self, state, anchor)
+		assert(Anchor:is(anchor), "can only resume to an anchor target") -- well technically it wouldn't be hard to allow to resume to any node, but I feel like there's already enough stuff in Anselme that was done just because it could be done
+		state.scope:push_partial(resume_anchor_identifier)
+		state.scope:define(resume_anchor_symbol, anchor)
+	end,
+	-- pop the current resume context
+	pop = function(self, state)
+		state.scope:pop()
+	end,
+
+	-- returns true if we are currently trying to resume to an anchor
+	resuming = function(self, state)
+		return state.scope:defined(resume_anchor_identifier) and not Nil:is(state.scope:get(resume_anchor_identifier))
+	end,
+	-- returns the anchor we are trying to resume to
+	get = function(self, state)
+		return state.scope:get(resume_anchor_identifier)
+	end,
+	-- mark the anchor as reached and stop the resume
+	set_reached = function(self, state)
+		state.scope:set(resume_anchor_identifier, Nil:new())
+	end,
+}
+
+package.loaded[...] = resume_manager
+
+Nil, Identifier, Anchor = ast.Nil, ast.Identifier, ast.Anchor
+
+resume_anchor_identifier = Identifier:new("_resume_anchor")
+resume_anchor_symbol = resume_anchor_identifier:to_symbol()
+
+return resume_manager