Pin fewer objects in engine.cpp

[d-netto]

#242 (comment) showed that we pin a lot of InferenceBenchmarkerCache in engine.cpp, probably to avoid moving objects whose pointers are stored in C++ data structures.

We have a lot of entries like this in the pinning profile:

  {
    "pinned_object": "0x200ffd5bce0",
    "type": "InferenceBenchmarkerCache",
    "pinning_sites": [
      {
        "filename": "/home/diogo/Code/MMTk/julia/src/engine.cpp",
        "lineno": 69,
        "count": 1
      },
      {
        "filename": "/home/diogo/Code/MMTk/julia/src/engine.cpp",
        "lineno": 142,
        "count": 1
      }
    ]
  },

We should find a way to optimize this so that we can move more InferenceBenchmarkerCache.

[d-netto]

Fixed by mmtk/julia#89.