Adds to nextString() method to ConfigStaticManifest

This is used to get the classname out of the tokens, rather than assuming that
the class name is a single T_STRING.

This allows for parsing of XHP classes, where the name `:a:b` gets turned into ":", T_STRING[a], ":", T_STRING[b].

It's a bit hard to write a test for this behaviour, as the only instances I've found of where it happens require PHP extensions.
 
[ref:XHPLOADER1]