SI-4989 Reject super.x if an intermediate class declares x abstract.

This is in line with Java's treatment. Without this, an AbstractMethodError
is thrown at runtime.
adds `narrow` to the reflection API

as per https://issues.scala-lang.org/browse/SI-5951
Fix SI-5284.

The problem was the false assumption that methods specialized
on their type parameter, such as this one:

    class Foo[@spec(Int) T](val x: T) {
      def bar[@spec(Int) S >: T](f: S => S) = f(x)
    }

have their normalized versions (`bar$mIc$sp`) never called
from the base specialization class `Foo`.
This meant that the implementation of `bar$mIc$sp` in `Foo`
simply threw an exception.

This assumption is not true, however. See this:

    object Baz {
      def apply[T]() = new Foo[T]
    }

Calling `Baz.apply[Int]()` will create an instance of the
base specialization class `Foo` at `Int`.
Calling `bar` on this instance will be rewritten by
specialization to calling `bar$mIc$sp`, hence the error.

So, we have to emit a valid implementation for `bar`,
obviously.
Problem is, such an implementation would have conflicting
type bounds in the base specialization class `Foo`, since
we don't know if `T` is a subtype of `S = Int`.
In other words, we cannot emit:

    def bar$mIc$sp(f: Int => Int) = f(x) // x: T

without typechecking errors.

However, notice that the bounds are valid if and only if
`T = Int`. In the same time, invocations of `bar$mIc$sp` will only
be emitted in callsites where the type bounds hold.
This means we can cast the expressions in method applications
to the required specialized type bound.

The following changes have been made:
1) The decision of whether or not to create a normalized
version of the specialized method is not done on the
`conflicting` relation anymore.
Instead, it's done based on the `satisfiable` relation,
which is true if there is possibly an instantiation of
the type parameters where the bounds hold.
2) The `satisfiable` method has a new variant called
`satisfiableConstraints`, which does unification to
figure out how the type parameters should be instantiated
in order to satisfy the bounds.
3) The `Duplicators` are changed to transform a tree
using the `castType` method which just returns the tree
by default.
In specialization, the `castType` in `Duplicators` is overridden,
and uses a map from type parameters to types.
This map is obtained by `satisfiableConstraints` from 2).
If the type of the expression is not equal to the expected type,
and this map contains a mapping to the expected type, then
the tree is cast, as discussed above.

Additional tests added.

Review by @dragos
Review by @VladUreche

Conflicts:

	src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
	src/compiler/scala/tools/nsc/typechecker/Duplicators.scala
Fix SI-4809.
Merge pull request #755 from axel22/issue/5284-cherry-picked

Fix SI-5284.
Exported new packages in the MANIFEST file.
Merge pull request #756 from axel22/issue/4809

Fix SI-4809.
SI-5966 Fix eta expansion for repeated parameters with zero arguments.

Reworks part of e33901 / SI-5610, which was inserting an <empty> tree
as an argument in this case, which turns into a null in icode.
SI-5968 Eliminate spurious exhaustiveness warning with singleton types.

A singleton type is a type ripe for enumeration.
Added a key comment.

The commit message from f4d2678c42 works a lot better up close
and personal with the change.
Adds missing closing curly brace to ScalaDoc code example.
IntMap: Made life a bit less scary for those doing source archaeology.
LongMap: Made life a bit less scary for those doing source archaeology.
Merge pull request #733 from retronym/ticket/4989-3

SI-4989 Reject super.x if an intermediate class declares x abstract.
Merge pull request #758 from dragos/update-manifest-file

Exported new packages in the MANIFEST file.
Merge pull request #761 from paulp/topic/parser-comment

Added a key comment.
Merge pull request #751 from scalamacros/topic/narrow

SI-5951 add `narrow` to the reflection API
Merge pull request #762 from soc/patch-3

Adds missing closing curly brace to ScalaDoc code example.
Merge pull request #764 from soc/patch-5

remove semi-colons and extraneous whitespace
Merge pull request #766 from soc/patch-8

remove semi-colons and extraneous whitespace