MethodHandles: Implement spreader methods.

Implements MethodHandle.asSpreader and MethodHandles.spreadInvoker.
This is the reverse of var-args collection, and the Transformer
implementation is symmetric with VarargsCollector (modulo inconsitencies
in the API).

Test: make test-art-host

Change-Id: If4deaa52f14143115b573bb0616e2eb72e83f859