Array copying instructions

[tlively]

The J2CL team asked whether there were plans for bulk-memory-like operations for arrays because they expect engines to be able to copy arrays faster than could be done in userspace. There is prior art in that Java's System.arraycopy is implemented as a native method:

https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/19fb8f93c59dfd791f62d41f332db9e306bc1422/src/hotspot/share/prims/jvm.cpp#L322-L335

https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/19fb8f93c59dfd791f62d41f332db9e306bc1422/src/hotspot/share/oops/typeArrayKlass.cpp#L125-L191

I am not aware of previous issues proposing this kind of instruction. It would be interesting to see how big the performance differences would be for large arrays, although this also seems like a good candidate for a follow-on proposal.

[tlively]

Closing this in light of #278