Maintenance of Predef.

1) Deprecates much of Predef and scala.Console, especially:

 - the read* methods (see below)
 - the set{Out,Err,In} methods (see SI-4793)

2) Removed long-deprecated:

 - Predef#exit
 - Predef#error should have gone, but could not due to sbt

At least the whole source base has now been future-proofed
against the eventual removal of Predef#error.

The low justification for the read* methods should be readily
apparent: they are little used and have no call to be in global
namespace, especially given their weird ad hoc semantics and
unreasonably tempting names such as readBoolean().

3) Segregated the deprecated elements in Predef from the part
which still thrives.

4) Converted all the standard Predef implicits into implicit
classes, value classes where possible:

  - ArrowAssoc, Ensuring, StringFormat, StringAdd, RichException (value)
  - SeqCharSequence, ArrayCharSequence (non-value)

Non-implicit deprecated stubs prop up the names of the
formerly converting methods.