Function luaX_newstring

** Creates a new string and anchors it in scanner's table so that it ** will not be collected until the end of the compilation; by that time ** it should be anchored somewhere. It also internalizes long strings, ** ensuring there is only one copy of each unique string. The table ** here is used as a set: the string enters as the key, while its value ** is irrelevant. We use the string itself as t