Move around some panics in wasmtime (#804)

In preparation for eventual support for wasm interface types this commit
moves around a few panics internally inside of conversions between the
`wasmtime` crate and the underlying jit support crates. This should have
any immediately-visible user changes, but the goal is that this'll help
support interface types which means `wasmtime` will have types that are
not supported by wasmtime itself and we'll be able to more gracefully
support that with error messages instead of accidental panics.

crates/api/src/instance.rs

@@ -132,7 +132,14 @@ impl Instance {
                 // imported into this store using the from_handle() method.
                 let _ = store.register_wasmtime_signature(signature);
             }
-            let extern_type = ExternType::from_wasmtime_export(&export);
+
+            // We should support everything supported by wasmtime_runtime, or
+            // otherwise we've got a bug in this crate, so panic if anything
+            // fails to convert here.
+            let extern_type = match ExternType::from_wasmtime_export(&export) {
+                Some(ty) => ty,
+                None => panic!("unsupported core wasm external type {:?}", export),
+            };
             exports_types.push(ExportType::new(name, extern_type));
             exports.push(Extern::from_wasmtime_export(
                 store,