tools/linedebug: Print each line of source executed

The extremely verbose debug output. Nice for getting some idea which
code runs most often, or where it pauses for no reason etc.

tools/linedebug.lua

@@ -0,0 +1,18 @@
+local data = {}
+local getinfo = debug.getinfo;
+local function linehook(ev, li)
+	local S = getinfo(2, "S");
+	if S and S.source and S.source:match"^@" then
+		local file = S.source:sub(2);
+		local lines = data[file];
+		if not lines then
+			lines = {};
+			data[file] = lines;
+			for line in io.lines(file) do
+				lines[#lines+1] = line;
+			end
+		end
+		io.stderr:write(ev, " ", file, " ", li, " ", lines[li], "\n");
+	end
+end
+debug.sethook(linehook, "l");