'not implemented' in `ObjectModel::copy` for header_offset 16

[qinsoon]

This seems to be non-deterministic.

JULIA julia-mmtk-immix-moving-release/usr/lib/julia/corecompiler.ji
thread '<unnamed>' panicked at src/object_model.rs:74:13:
not implemented
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The panic happens here

mmtk-julia/mmtk/src/object_model.rs

Lines 72 to 75 in 33ddc1d

	} else if header_offset == 16 {
	// buffer should not be copied
	unimplemented!();
	} else {

[qinsoon]

We still see this occasionally. We intend to pin all the buffers, but it seems there are buffers that are not pinned, and get copied by the GC.