String functions

I added Yuji Minejima's string tests from Sacla, and then fixed/wrote string functions for those that were missing.  (I guess I could have just added his, but this way was more fun.)  There's still a few which are missing (mostly related to the different character types), and I put some comments in there as to what they need.

There's also POSITION-IF, which I thought would be an easy way to write STRING-TRIM, but it turns out you really need :FROM-END for that, and that's not as easy to do efficiently.