[indylambda] Relieve LambdaMetafactory of boxing duties

`LambdaMetafactory` generates code to perform a limited number
of type adaptations when delegating from its implementation of
the functional interface method to the lambda target method.

These adaptations are: numeric widening, casting, boxing and unboxing.

However, the semantics of unboxing numerics in Java differs to Scala:
they treat `UNBOX(null)` as cause to raise a `NullPointerException`,
Scala (in `BoxesRuntime.unboxTo{Byte,Short,...}`) reinterprets the
null as zero.

Furthermore, Java has no idea how to adapt between a value class and
its wrapped type, nor from a void return to `BoxedUnit`.

This commit detects when the lambda target method would require
such adaptation. If it does, an extra method, `$anonfun$1$adapted` is
created to perform the adaptation, and this is used as the target
of the lambda.

This obviates the use of `JProcedureN` for `Unit` returning
lambdas, we know use `JFunctionN` as the functional interface
and bind this to an `$adapted` method that summons the instance
of `BoxedUnit` after calling the `void` returning lambda target.

The enclosed test cases fail without boxing changes. They don't
execute with indylambda enabled under regular partest runs yet,
you need to add scala-java8-compat to scala-library and pass
the SCALAC_OPTS to partest manually to try this out, as described
in https://github.com/scala/scala/pull/4463. Once we enable indylambda
by default, however, this test will exercise the code in this patch
all the time.

It is also possible to run the tests with:

```
% curl https://oss.sonatype.org/content/repositories/releases/org/scala-lang/modules/scala-java8-compat_2.11/0.4.0/scala-java8-compat_2.11-0.4.0.jar > scala-java8-compat_2.11-0.4.0.jar
% export INDYLAMBDA="-Ydelambdafy:method -Ybackend:GenBCode -target:jvm-1.8 -classpath .:scala-java8-compat_2.11-0.4.0.jar"
qscalac $INDYLAMBDA test/files/run/indylambda-boxing/*.scala && qscala $INDYLAMBDA Test
```

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