use stringer/strcat instead of str for all of our string composition

[NBKelly]

I went through (almost) every backend class to do this. Essentially, java is sloppy and slow with string operations using the basic libraries, and clojure inherits this. Stringer is a bit more optimized, so this is basically a "free" speedup (do note that the compile time will be a little longer, but we don't care too much about that).