moves compileTimeOnly to scala-library

This is the notion that's come to be universally useful, so I suggest
we promote it to be universally accessible.

Note that the attached test incorrectly fails to report errors for
definitions coming from the empty package and for annotations. These
are bugs, and they are fixed in subsequent commits of this pull request.
marks Expr.splice and Expr.value with @compileTimeOnly

Now that @compileTimeOnly is part of the standard library, why don't
we use it within the standard library.
@compileTimeOnly now works for annotations

http://docs.scala-lang.org/overviews/macros/annotations.html
say sincere "thank you!".
@compileTimeOnly now works for symbols from the empty package

Looks like we've got the entire language covered now.