Change Buffer in SimpleInterpreter to use a memoryboard

[jsbueno]

Currently, instantiating a interpreter will use a fixed-size buffer, with fixed offsets to pass serialized objects back and forth. The default size is too big for most use cases, and create a memory overhead larger than that of a separate interpreter itself.

The more structured "memoryboard" can be used for this role, with a smaller overhead and more robustness.