SI-7965 Support calls to MethodHandle.{invoke,invokeExact}

These methods are "signature polymorphic", which means that compiler
should not:
  1. adapt the arguments to `Object`
  2. wrap the repeated parameters in an array
  3. adapt the result type to `Object` if the enclosing expression
     is a cast or if the application appears in the statement position
     of a block.

Dispiritingly, my initial attempt to implement this touched the type
checker, uncurry, erasure, and the backend.

However, I realized we could centralize handling of this in the typer
if at each application we substituted the signature polymorphic
symbol with a clone that carried its implied signature, which is
derived from the types of the arguments (typechecked without an
expected type) and position within and enclosing cast or block.

The test case requires Java 7+ to compile so is currently embedded
in a conditionally compiled block of code in a run test.

We ought to create a partest category for modern JVMs so we can
write such tests in a more natural style.

Here's how this looks in bytecode. Note the `bipush` / `istore`
before/after the invocation of `invokeExact`, and the descriptor
`(LO$;I)I`.

```
% cat sandbox/poly-sig.scala && qscala Test && echo ':javap Test$#main' | qscala
import java.lang.invoke._

object O {
  def bar(x: Int): Int = -x
}

object Test {
  def main(args: Array[String]): Unit = {
    def lookup(name: String, params: Array[Class[_]], ret: Class[_]) = {
      val lookup = java.lang.invoke.MethodHandles.lookup
      val mt = MethodType.methodType(ret, params)
      lookup.findVirtual(O.getClass, name, mt)
    }
    def lookupBar = lookup("bar", Array(classOf[Int]), classOf[Int])

    val barResult = lookupBar.invokeExact(O, 42).asInstanceOf[Int]
    ()
  }
}

scala> :javap Test$#main
  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=2
         0: aload_0
         1: invokespecial #18                 // Method lookupBar$1:()Ljava/lang/invoke/MethodHandle;
         4: getstatic     #23                 // Field O$.MODULE$:LO$;
         7: bipush        42
         9: invokevirtual #29                 // Method java/lang/invoke/MethodHandle.invokeExact:(LO$;I)I
        12: istore_2
        13: return
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      14     0  this   LTest$;
            0      14     1  args   [Ljava/lang/String;
           13       0     2 barResult   I
      LineNumberTable:
        line 16: 0
}
```

I've run this test across our active JVMs:

```
% for v in 1.6 1.7 1.8; do java_use $v; pt --terse test/files/run/t7965.scala || break; done
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)
Selected 1 tests drawn from specified tests

.

1/1 passed (elapsed time: 00:00:02)
Test Run PASSED
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)
Selected 1 tests drawn from specified tests

.

1/1 passed (elapsed time: 00:00:07)
Test Run PASSED
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)
Selected 1 tests drawn from specified tests

.

1/1 passed (elapsed time: 00:00:05)
Test Run PASSED
```
`.filter(_.exists)` instead of `.filterNot(_ == NoSymbol)`