Avoid use of classes forbidden by Google App Engine

Google App Engine does not allow certain classes and methods, including some that instaparse uses. This pull request makes two changes that allow web apps that use instaparse to be hosted on App Engine:

(1) In core.clj, if a string is very long assume it is not a filename, and do not make file I/O calls.
(2) In gll.clj, do not use the forbidden javax.swing.text.Segment class, but instead use deftype to create a similar class, that I believe is similarly efficient.

The "lein test" still passes, as do extensive exercising test in my own app.

I did a performance test, and found there was no degradation of speed.