Remove dependency on xml in sys.process.

Added deprecated conversions in xml.Elem to preserve source compatibility.
Remove dependency on xml in ScalaRunTime.

runtime.ScalaRunTime.stringOf's behavior has changed slightly:
Call the object's `toString` for all classes in `scala.xml._`,
instead of only for `Node` and `MetaData` (old behavior).
Remove dependency on xml in plugin loading.
Remove dependency on xml in MarkupParsers

Inlined TextBuffer.
Remove dependency on xml in ast.parser

Copied the following files from scala.xml to ast.parser.xml:
  - MarkupParsers differs from scala.xml.MarkupParsers only in the first 17 lines
  - Utility.scala was refactored and reduced (also includes TokenTests)
Set scene for Predef.$scope's demise.

When there's no Predef.$scope but xml is being used,
the compiler aliases scala.xml.TopScope to $scope.

There must be a scala.xml package when xml literals were parsed.

For compatibility with the old library, which relied on $scope being in scope,
synthesize a `import scala.xml.{TopScope => $scope}` when xml is needed,
but there's no Predef.$scope and the old library is detected (scala.xml.TopScope exists).
Merge pull request #2667 from adriaanm/modularize-xml

Spin off xml library as scala-library-xml.