Create simple-format module, move format-to-string.

This moves the format-to-string binding from common-extensions
to a simple-format module which will be replacing simple-io.
Project wizard knows about simple-format instead of simple-io.
[make-dylan-app] No longer need to exclude format-to-string.
Merge pull request #751 from waywardmonkeys/move-format-to-string

Move format-to-string