Add strict module

Also fix leaking globals found by it.

builtin/common/strict.lua

@@ -0,0 +1,47 @@
+
+-- Always warn when creating a global variable, even outside of a function.
+-- This ignores mod namespaces (variables with the same name as the current mod).
+local WARN_INIT = false
+
+
+local function warn(message)
+	print(os.date("%H:%M:%S: WARNING: ")..message)
+end
+
+
+local meta = {}
+local declared = {}
+
+
+function meta:__newindex(name, value)
+	local info = debug.getinfo(2, "Sl")
+	local desc = ("%s:%d"):format(info.short_src, info.currentline)
+	if not declared[name] then
+		if info.what ~= "main" and info.what ~= "C" then
+			warn(("Assignment to undeclared global %q inside"
+					.." a function at %s.")
+				:format(name, desc))
+		end
+		declared[name] = true
+	end
+	-- Ignore mod namespaces
+	if WARN_INIT and (not core.get_current_modname or
+			name ~= core.get_current_modname()) then
+		warn(("Global variable %q created at %s.")
+			:format(name, desc))
+	end
+	rawset(self, name, value)
+end
+
+
+function meta:__index(name)
+	local info = debug.getinfo(2, "Sl")
+	if not declared[name] and info.what ~= "C" then
+		warn(("Undeclared global variable %q accessed at %s:%s")
+				:format(name, info.short_src, info.currentline))
+	end
+	return rawget(self, name)
+end
+
+setmetatable(_G, meta)
+