Remove redundant type assertions in lua engine

[adatzer]

The type assertions in the switch cases here and here could be removed by using the result of the type switch.

For example:

// before
switch val.(type) {
case float64:
	ltbl.RawSetString(key, lua.LNumber(val.(float64)))
case int64:
	ltbl.RawSetString(key, lua.LNumber(val.(int64)))
// ...

// after
switch valTyped := val.(type) {
case float64:
	ltbl.RawSetString(key, lua.LNumber(valTyped))
case int64:
	ltbl.RawSetString(key, lua.LNumber(valTyped))
// ...