Refactor (#1524)

* Compute instance exports on demand.

Instead having instances eagerly compute a Vec of Externs, and bumping
the refcount for each Extern, compute Externs on demand.

This also enables `Instance::get_export` to avoid doing a linear search.

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.

* Compute module imports and exports on demand too.

And compute Extern::ty on demand too.

* Add a utility function for computing an ExternType.

* Add a utility function for looking up a function's signature.

* Add a utility function for computing the ValType of a Global.

* Rename wasmtime_environ::Export to EntityIndex.

This helps differentiate it from other Export types in the tree, and
describes what it is.

* Fix a typo in a comment.

* Simplify module imports and exports.

* Make `Instance::exports` return the export names.

This significantly simplifies the public API, as it's relatively common
to need the names, and this avoids the need to do a zip with
`Module::exports`.

This also changes `ImportType` and `ExportType` to have public members
instead of private members and accessors, as I find that simplifies the
usage particularly in cases where there are temporary instances.

* Remove `Instance::module`.

This doesn't quite remove `Instance`'s `module` member, it gets a step
closer.

* Use a InstanceHandle utility function.

* Don't consume self in the `Func::get*` methods.

Instead, just create a closure containing the instance handle and the
export for them to call.

* Use `ExactSizeIterator` to avoid needing separate `num_*` methods.

* Rename `Extern::func()` etc. to `into_func()` etc.

* Revise examples to avoid using `nth`.

* Add convenience methods to instance for getting specific extern types.

* Use the convenience functions in more tests and examples.

* Avoid cloning strings for `ImportType` and `ExportType`.

* Remove more obviated clone() calls.

* Simplify `Func`'s closure state.

* Make wasmtime::Export's fields private.

This makes them more consistent with ExportType.

* Fix compilation error.

* Make a lifetime parameter explicit, and use better lifetime names.

Instead of 'me, use 'instance and 'module to make it clear what the
lifetime is.

* More lifetime cleanups.

crates/c-api/src/func.rs

@@ -246,7 +246,7 @@ fn _wasmtime_func_call(
 
 #[no_mangle]
 pub extern "C" fn wasm_func_type(f: &wasm_func_t) -> Box<wasm_functype_t> {
-    Box::new(wasm_functype_t::new(f.func().borrow().ty().clone()))
+    Box::new(wasm_functype_t::new(f.func().borrow().ty()))
 }
 
 #[no_mangle]
@@ -272,10 +272,10 @@ pub unsafe extern "C" fn wasmtime_caller_export_get(
     let name = str::from_utf8(name.as_slice()).ok()?;
     let export = caller.caller.get_export(name)?;
     let which = match export {
-        Extern::Func(f) => ExternHost::Func(HostRef::new(f.clone())),
-        Extern::Global(g) => ExternHost::Global(HostRef::new(g.clone())),
-        Extern::Memory(m) => ExternHost::Memory(HostRef::new(m.clone())),
-        Extern::Table(t) => ExternHost::Table(HostRef::new(t.clone())),
+        Extern::Func(f) => ExternHost::Func(HostRef::new(f)),
+        Extern::Global(g) => ExternHost::Global(HostRef::new(g)),
+        Extern::Memory(m) => ExternHost::Memory(HostRef::new(m)),
+        Extern::Table(t) => ExternHost::Table(HostRef::new(t)),
     };
     Some(Box::new(wasm_extern_t { which }))
 }