Fix ticket:53 by adding UTF-8 to the core.

code/extfmts.lisp:
* Move the utf-8 implementation to this file
* Update %find-external-format to return quickly for :utf-8. 

code/fd-stream-comp.lisp:
* Precompile utf-8 functions.

pcl/simple-streams/external-formats/utf-8.lisp:
* Add note that this is only used for reference now because it's in
  the core.