Modularized the repl.

Following in the footsteps of scaladoc and interactive.
The interpreter sources move into src/repl, and are given
a separate build target. As with the others, at present
they are still packaged into scala-compiler.jar.

A summary of changes:

 - repl requires use of ReplGlobal (this was already implied)
 - macro code's repl-specific classloader hack pulled into overridable
   method and overridden in ReplGlobal
 - removed -Ygen-javap option to eliminate backend's dependency on javap
 - removed -Yrepl-debug option (can still be enabled with -Dscala.repl.debug)
 - pushed javap code into src/repl so javax.tools dependency can bee
   weakened to the repl only
 - removed some "show pickled" related code which hasn't worked right
   in a while and isn't the right way to do it anymore anyway. Will
   return to fix showPickled and provide it with some tests.