Add a sandbox builder

[jprendes]

No description provided.

[syntactically]

I would prefer to move to a model where host functions are known at compile time & sandbox configuration is greatly reduced, so basically instead of all this you would just have create_snapshot_from_binary(binary), sandbox::new() (where host functions get registered at compile time, so that we don't have to do string comparisons constantly), load_snapshot_into_sandbox(snapshot, scratch_mem_size)