Map setmultispec op for MoarVM.
Add is_thunk to QAST::Block.

For now just as another attribute; there's now 3 of these that could
flatten into a single attribute of flag bits.
Pass is_thunk on down to Moar backend.
Map ctx[caller|outer]skipthunks for MoarVM.
Bump MOAR_REVISION to one with multispec.

This also happens to get spesh.
is_thunk and ctx[caller|outer]skipthunks on Parrot
is_thunk and ctx[caller|outer]skipthunks on JVM