lua: add a function that formats Lua values as strings

Yep, Lua is so crappy that the stdlib doesn't provide anything like
this.

Repurposes the undocumented mp.format_table() function and moves it to
mp.utils.

DOCS/man/lua.rst

@@ -623,6 +623,10 @@ strictly part of the guaranteed API.
     trailing text is returned as 3rd return value. (The 3rd return value is
     always there, but with ``trail`` set, no error is raised.)
 
+``utils.to_string(v)``
+    Turn the given value into a string. Formats tables and their contents. This
+    doesn't do anything special; it is only needed because Lua is terrible.
+
 Events
 ------
 

TOOLS/lua/observe-all.lua

@@ -1,16 +1,10 @@
 -- Test script for property change notification mechanism.
 
-function format_property_val(v)
-    if type(v) == "table" then
-        return mp.format_table(v) -- undocumented function; might be removed
-    else
-        return tostring(v)
-    end
-end
+local utils = require("mp.utils")
 
 for i,name in ipairs(mp.get_property_native("property-list")) do
     mp.observe_property(name, "native", function(name, val)
         print("property '" .. name .. "' changed to '" ..
-              format_property_val(val) .. "'")
+              utils.to_string(val) .. "'")
     end)
 end

player/lua/defaults.lua

@@ -485,7 +485,9 @@ function mp.add_hook(name, pri, cb)
     mp.commandv("hook_add", name, id, pri)
 end
 
-function mp.format_table(t, set)
+local mp_utils = package.loaded["mp.utils"]
+
+function mp_utils.format_table(t, set)
     if not set then
         set = { [t] = true }
     end
@@ -508,30 +510,33 @@ function mp.format_table(t, set)
             vals[#keys] = v
         end
     end
-    local function fmtval(v)
-        if type(v) == "string" then
-            return "\"" .. v .. "\""
-        elseif type(v) == "table" then
-            if set[v] then
-                return "[cycle]"
-            end
-            set[v] = true
-            return mp.format_table(v, set)
-        else
-            return tostring(v)
-        end
-    end
     for i = 1, #keys do
         if #res > 1 then
             res = res .. ", "
         end
         if i > arr then
-            res = res .. fmtval(keys[i]) .. " = "
+            res = res .. mp_utils.to_string(keys[i], set) .. " = "
         end
-        res = res .. fmtval(vals[i])
+        res = res .. mp_utils.to_string(vals[i], set)
     end
     res = res .. "}"
     return res
 end
 
+function mp_utils.to_string(v, set)
+    if type(v) == "string" then
+        return "\"" .. v .. "\""
+    elseif type(v) == "table" then
+        if set then
+            if set[v] then
+                return "[cycle]"
+            end
+            set[v] = true
+        end
+        return mp_utils.format_table(v, set)
+    else
+        return tostring(v)
+    end
+end
+
 return {}