SI-6899, prohibit dangerous, useless implicit conversions.

Increase eligibility requirements for implicit conversions,
such that T => U is ineligible if

  T <: Null    <or>   AnyRef <: U

This has the salutary effect of allowing us to ditch 16
ridiculous implicits from Predef, since they existed solely
to work around the absence of this restriction.

There was one tiny impact on actual source code (one line
in one file) shown here, necessitated because the literal null
is not eligible to be implicitly converted to A via <:<.

  def f[A](implicit ev: Null <:< A): A = null     // before
  def f[A](implicit ev: Null <:< A): A = ev(null) // after

As impositions go it's on the tame side.
SI-5022 Retain precise existentials through pattern matching

From the dawn of scalac's existentials, the typer widens
existentials pt-s by substituting wildcard types in places
of existential quantifiers.

In this example:

    class ForSomeVsUnapply {
      def test {
        def makeWrap: Wrap = ???
        def useRep[e](rep: (e, X[e])) = ()

        val rep = makeWrap match {
          case Wrap(r) => r
        };

        useRep(rep) // error
      }
    }

the type of `r` is the result of typechecking:

    Apply(
     fun = TypeTree(
       tpe = (rep#12037: (e#12038, X#7041[e#12038]) forSome { type e#12038 })
     args = Bind(r @ _) :: Nil
    }

This descends to type the `Bind` with:

    pt = (e#12038, X#7041[e#12038]) forSome { type e#12038 }

`dropExistential` clobbers that type to `Tuple2#1540[?, X#7041[?]]`,
which doesn't express any relationship between the two instances
of the wildcard type. `typedIdent` sort of reverses this with a call
to `makeFullyDefined`, but only ends up with:

    pt = (Any#3330, X#7041[_1#12227]) forSome { type _1#12227; type e#12038 }

I suspect that this existential dropping only makes sense outside of
typechecking patterns. In pattern mode, type information flows from the
expected type onwards to the body of the case; we must not lose precision
in the types.

For SIP-18 friendly existentials, one `dropExistential` is invertable with
`makeFullyDefined`, so this hasn't been such a big problem.

The error message improvement conferred by SI-4515 took a hit.
That might be a good example to consider when reviewing this change:
Does it tell us anything interesting about this `dropExistential`
business?
SI-7479 Add avian option to partest's diff filter...

... so that the different results of the test on Avian
can be specified in the check file.
SI-7151 Emit final in bytecode for final inner classes.

As we did before a regression in 18efdedfb / SI-5676.

This commit tightens up the condition in which the FINAL
modifier is omitted; it now *only* does this for the module
classes of nested objects.
SI-7364 Allow raw types in parent position in Java sources

To make this work, this commit simply restricts parent
type argument inference to Scala source files.

The surrounding code has also been refactored to avoid a var.
Add Duration.toCoarsest method
SI-7479 Make test/files/run/tailcalls.scala pass on Avian

This commit modifies the test and check file to use the new diff filter.
Add support for JUnit tests

Add `test.junit` ant target that compiles and runs JUnit tests
found in `test/junit` directory.

Add `scala.tools.nsc.SampleTest` that demonstrates working
testing infrastructure.
Add Eclipse project for JUnit tests.
Merge remote-tracking branch 'scala-2.10.x/junit' into master
Move WeakHashSetTest to junit tests.

The WeakHashSetTest was written as unit test but put into partest's
`run` category as we were missing direct unit testing support.
That got fixed so moving the test now.
Merge pull request #2561 from soc/SI-7479

SI-7479 Make test/files/run/tailcalls.scala pass on Avian
Merge pull request #2602 from t3hnar/Duration.toCoarsest

Add Duration.toCoarsest method
Merge pull request #2657 from gkossakowski/junit-master

Merge JUnit support into master
Merge pull request #2631 from retronym/ticket/5022

SI-5022 Retain precise existentials through pattern matching
Merge pull request #2635 from retronym/ticket/7151

SI-7151 Emit final in bytecode for final inner classes.
Merge pull request #2628 from retronym/ticket/7364

SI-7364 Allow raw types in parent position in Java sources
Merge pull request #2625 from paulp/pr/no-useless-implicits

SI-6899, prohibit dangerous, useless implicit conversions.
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).