[sammy] eta-expansion, overloading (SI-8310)

Playing with Java 8 Streams from the repl showed
we weren't eta-expanding, nor resolving overloading for SAMs.

Also, the way Java uses wildcards to represent use-site variance
stresses type inference past its bendiness point (due to excessive existentials).

I introduce `wildcardExtrapolation` to simplify the resulting types
(without losing precision): `wildcardExtrapolation(tp) =:= tp`.

For example, the `MethodType` given by `def bla(x: (_ >: String)): (_ <: Int)`
is both a subtype and a supertype of `def bla(x: String): Int`.

Translating http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/
into Scala shows most of this works, though we have some more work to do (see near the end).

```
scala> import java.util.Arrays
scala> import java.util.stream.Stream
scala> import java.util.stream.IntStream
scala> val myList = Arrays.asList("a1", "a2", "b1", "c2", "c1")
myList: java.util.List[String] = [a1, a2, b1, c2, c1]

scala> myList.stream.filter(_.startsWith("c")).map(_.toUpperCase).sorted.forEach(println)
C1
C2

scala> myList.stream.filter(_.startsWith("c")).map(_.toUpperCase).sorted
res8: java.util.stream.Stream[?0] = java.util.stream.SortedOps$OfRef@133e7789

scala> Arrays.asList("a1", "a2", "a3").stream.findFirst.ifPresent(println)
a1

scala> Stream.of("a1", "a2", "a3").findFirst.ifPresent(println)
a1

scala> IntStream.range(1, 4).forEach(println)
<console>:37: error: object creation impossible, since method accept in trait IntConsumer of type (x$1: Int)Unit is not defined
(Note that Int does not match Any: class Int in package scala is a subclass of class Any in package scala, but method parameter types must match exactly.)
              IntStream.range(1, 4).forEach(println)
                                            ^

scala> IntStream.range(1, 4).forEach(println(_: Int)) // TODO: can we avoid this annotation?
1
2
3

scala> Arrays.stream(Array(1, 2, 3)).map(n => 2 * n + 1).average.ifPresent(println(_: Double))
5.0

scala> Stream.of("a1", "a2", "a3").map(_.substring(1)).mapToInt(_.parseInt).max.ifPresent(println(_: Int)) // whoops!
ReplGlobal.abort: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
error: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
scala.reflect.internal.FatalError: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
	at scala.reflect.internal.Reporting$class.abort(Reporting.scala:59)

scala> IntStream.range(1, 4).mapToObj(i => "a" + i).forEach(println)
a1
a2
a3

```
[sammy] use correct type for method to override

Don't naively derive types for the single method's signature
from the provided function's type, as it may be a subtype
of the method's MethodType.

Instead, once the sam class type is fully defined, determine
the sam's info as seen from the class's type, and use those
to generate the correct override.

```
scala> Arrays.stream(Array(1, 2, 3)).map(n => 2 * n + 1).average.ifPresent(println)
5.0

scala> IntStream.range(1, 4).forEach(println)
1
2
3
```

Also, minimal error reporting

Can't figure out how to do it properly, but some reporting
is better than crashing. Right? Test case that illustrates
necessity of the clumsy stop gap `if (block exists (_.isErroneous))`
enclosed as `sammy_error_exist_no_crash`

added TODO for repeated and by-name params
[sammy] support repeated params

Generate correct trees to refer to repeated params using `gen.paramToArg`.
Based on retronym's review feedback.
Merge pull request #4101 from adriaanm/sam-ex

[sammy] eta-expansion, overloading, existentials