Implement the global storage operators (mock-ups).

[nvjle]

The compiler has progressed to the point where we will soon need to interface with the Solana data model-- the persistent store as Move calls it. That work has not yet begun and depends on bcs. For the time being, implement mock versions of the global storage operators (e.g., move_to) so that we can start doing wider testing.

These operators are already mentioned in https://github.com/solana-labs/move/issues/75. This issue is slightly different in that we want a short-term way to unit-test global storage operators before we have the ability to connect to an actual Solana ledger (and before we have bcs).