This fixes the case where the old name is used before the new one
is ever used, e.g. when manually constructing nodes, as opposed to
parsing them.
The previous approach would try to register the alias from OLD to
NEW. This would trigger autoloading on NEW and afterwards it would
register the alias from OLD to NEW. Afterwards the alias registration
which originally triggered the autoload would run, thus redeclaring
the class.
TL;DR aliases suck, closes#192.
Running a .phar or regular PHP executable that requires and includes its own
version of php-parser will cause a "cannot redeclare class" error if said
executable also includes the autoloader of the current working directory.
