Avoid storing source file contents twice

BatchSourceFile instances contain two almost identical character arrays,
content0 and content. What's worse, content0 is even public. Instead,
content0 should just be a constructor parameter. This seems the residual
of an incomplete refactoring.

I observed this waste of memory during debugging; after applying this
patch, I've verified by hand that the second field indeed disappears.

I don't expect a measurable difference, but this patch is not premature
optimization because it makes code more logical.
Revert "SI-5920 enables default and named args in macros"

This reverts commit a02e053a5dec134f7c7dc53a2c1091039218237d.

That commit lead to an error compiling Specs2:

    [info] [warn] /localhome/jenkinsdbuild/workspace/Community-2.11.x-retronym/dbuild-0.7.1-M1/target-0.7.1-M1/project-builds/specs2-aaa8091b47a34817ca90134ace8b09a9e0f854e9/core/src/test/scala/org/specs2/text/EditDistanceSpec.scala:6: Unused import
    [info] [warn] import DiffShortener._
    [info] [warn]                      ^
    [info] [error] /localhome/jenkinsdbuild/workspace/Community-2.11.x-retronym/dbuild-0.7.1-M1/target-0.7.1-M1/project-builds/specs2-aaa8091b47a34817ca90134ace8b09a9e0f854e9/core/src/test/scala/org/specs2/text/LinesContentDifferenceSpec.scala:7: exception during macro expansion:
    [info] [error] java.lang.UnsupportedOperationException: Position.point on NoPosition
    [info] [error]  at scala.reflect.internal.util.Position.fail(Position.scala:53)
    [info] [error]  at scala.reflect.internal.util.UndefinedPosition.point(Position.scala:131)
    [info] [error]  at scala.reflect.internal.util.UndefinedPosition.point(Position.scala:126)
    [info] [error]  at org.specs2.reflect.Macros$.sourceOf(Macros.scala:25)
    [info] [error]  at org.specs2.reflect.Macros$.stringExpr(Macros.scala:19)
Merge pull request #3542 from retronym/revert/5920

Revert "SI-5920 enables default and named args in macros"
Merge pull request #3539 from Blaisorblade/topic/performance

Avoid storing source file contents twice