Rewrite calls of meta_exec with new structure

- Pass a table to meta_exec containing parameters, function to be
   called, scope string and requirements to check
 - Add basic ownership and contextid requirement checks in meta_exec
   when needed
 - Update README.md
 - Move assertion tools to assertions.lua

assertions.lua

@@ -0,0 +1,38 @@
+-- Assertion tools for Metatools
+
+function assert_contextid(ctid)
+	return contexts[ctid] ~= nil
+end
+
+function assert_ownership(ctid, name)
+	return playerlocks[name] == ctid
+end
+
+function assert_pos(pos)
+	if type(pos) ~= "string" then
+		return pos and pos.x and pos.y and pos.z and minetest.pos_to_string(pos)
+	else
+		return minetest.string_to_pos(pos) ~= nil
+	end
+end
+
+function assert_mode(mode)
+	return mode and (mode == "fields" or mode == "inventory")
+end
+
+function assert_poslock(pos)
+	return nodelock[minetest.pos_to_string(pos)] == nil
+end
+
+function assert_specific_mode(contextid, mode)
+	return assert_contextid(contextid) and contexts[contextid].mode == mode
+end
+
+function assert_field_type(ftype)
+	return ftype and type(ftype) == "string" and (ftype == "int" or ftype == "float" or ftype == "string")
+end
+
+function assert_integer(int)
+	return int and tonumber(int) and tonumber(int) % 1 == 0
+end
+