Rename the enable_collection exposed by the binding

[caizixian]

#88 calls the correct method after changed in mmtk/mmtk-core#474. However, we should rename the enable_collection function (and create a new JikesRVM syscall) as well to avoid confusion.

[qinsoon]

I suggest we keep the name enableCollection/enable_collection() in JikesRVM and inevitably in the binding as well. It is used in JikesRVM in the following code:

    if (VM.BuildWithRustMMTk) {
      sysCall.sysEnableCollection(Magic
              .objectAsAddress(RVMThread.getCurrentThread()));
    } else {
      Selected.Plan.get().enableCollection();
    }

I think we want to keep the third party call consistent with the Java MMTk call. So think about it in this way: JikesRVM requires the heap implementation to provide a way to enable collection (for third party heap and for Java MMTk), and in the binding, we implement the enable collection call for JikesRVM, and we call initialize_collection() in mmtk core.