Promise resolution/synchronization

[smarr]

Currently, promises synchronize on the potentially wrapped value: https://github.com/smarr/SOMns/blob/master/src/som/interpreter/actors/SPromise.java#L343

Since these values can also be primitives, it could be highly problematic, and potentially deadlock prone, in case primitives like ints map to the same boxed object for promises that are to be resolved in parallel.

Need to properly characterize the synchronization dependencies here again, and figure out whether this is an actual issue.

This was identified as part of https://github.com/graalvm/graal-core/issues/201

[smarr]

This is needed for full SVM support #166.