This adds support for the WASI reactor ABI proposed in
WebAssembly/WASI#256, as well as the existing
_start ABI for commands.

This also implements the semantics that command instances should not
persist after their _start function is called. As a safety measure,
this includes a check that the instance refcount is 1, so that other
references to the instance don't hold it live.

Implementing that prompted a change to how the main Instance API works.
Instead having instances eagerly compute a Vec of Externs, and bumping
the refcount for each Extern, compute Externs on demand.

This also means that the closure returned by `get0` and friends now
holds an `InstanceHandle` to dynamically hold the instance live rather
than being scoped to a lifetime.

Make `Module::exports` and `Module::imports` return iterators instead of
slices, and compute them on demand rather than eagerly.

Also, compute them from wasmtime_environ's data structures, rather than
re-decoding the wasm bytes.

To support this, change wasmtime_environ::Module hold a single imports
array rather than having per-type arrays.