Hackatom: Pass storage in context

[ethanfrey]

When running integration tests, we used a singleton Storage to mock out the system calls for do_read and do_write. After doing some experiments in setup, I discovered how to pass into into context, available on a per-instance basis. Let's remove this singleton.

Relevant info:

• General discussion: Using wasmer with context wasmerio/wasmer-rust-example#4
• Use of closure in imports: Remove ctx argument on imported functions and allow closures. wasmerio/wasmer#219 (comment)

[ethanfrey]

Closed with 774f093