Implement Memory Snapshotting and Restore

[simongdavies]

A lot of the work that is done in initialization of a sandbox could potentially be done offline out side the hot path, likewise there may be cases where a host process wants or needs to capture the state of a sandbox.

We should design and implement a mechanism to allow a host to save and restore consistent snapshots of memory (and other state required) to allow Sandboxes to be efficiently persisted/initialised/started.