* Fix a possible use-after-free with `Global`
This commit fixes an issue with the implementation of the
`wasmtime::Global` type where if it previously outlived the original
`Instance` it came from then you could run into a use-after-free. Now
the `Global` type holds onto its underlying `InstanceHandle` to ensure
it retains ownership of the underlying backing store of the global's
memory.
* rustfmt
