refactor(lua): use Arena when converting from lua stack to API args

and for return value of nlua_exec/nlua_call_ref, as this uses
the same family of functions.

NB: the handling of luaref:s is a bit of a mess.
add api_luarefs_free_XX functions as a stop-gap as refactoring
luarefs is a can of worms for another PR:s.

as a minor feature/bug-fix, nvim_buf_call and nvim_win_call now preserves
arbitrary return values.

test/functional/lua/vim_spec.lua

@@ -3543,6 +3543,18 @@ describe('lua stdlib', function()
       ]])
       )
     end)
+
+    it('can return values by reference', function()
+      eq(
+        { 4, 7 },
+        exec_lua [[
+        local val = {4, 10}
+        local ref = vim.api.nvim_buf_call(0, function() return val end)
+        ref[2] = 7
+        return val
+      ]]
+      )
+    end)
   end)
 
   describe('vim.api.nvim_win_call', function()
@@ -3640,6 +3652,18 @@ describe('lua stdlib', function()
                                       |
       ]]
     end)
+
+    it('can return values by reference', function()
+      eq(
+        { 7, 10 },
+        exec_lua [[
+        local val = {4, 10}
+        local ref = vim.api.nvim_win_call(0, function() return val end)
+        ref[1] = 7
+        return val
+      ]]
+      )
+    end)
   end)
 
   describe('vim.iconv', function()