Create a new multibinder alternate for Set<? extends T>

[swankjesse]

This is a simple linked binding to the original multibinder set. This is
safe because 'Set<? extends T>' is assignable from 'Set'.

This is useful for projects integrating Guice and Kotlin. In Kotlin there
are different types for read-only vs. read-write Sets. When declaring a
read-only Set, the bytecode includes a wildcard.

In particular, this Kotlin:

var setA: Set<Runnable>
var setB: MutableSet<Runnable>

is equivalent to this Java:

Set<? extends Runnable> setA;
Set<Runnable> setB;

Using Kotlin with Guice multibindings is quite annoying because the binding
to Set<Runnable> is typically provided, but Set<? extends Runnable> is
needed.

The typical Kotlin workaround is also ugly:

var setA: Set<@JvmSuppressWildcards Runnable>

With this fix, Guice multibindings and Guice should just work.

I expect this may be backwards-incompatible with a very small number of Kotlin
applications that implement a different workaround to the above problem:

@Provides
Set<? extends Runnable> provideRunnables(Set<Runnable> set) {
  return set;
}

I don't expect backwards incompatibility with Java programs. In such programs
the behavior change will result in a fast failure (injector fails to create)
which is easy to diagnose and fix.

Closes: #1282