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/obj/src/context.rs

@@ -42,7 +42,7 @@ pub fn layout_vmcontext(
         }
     }
 
-    let num_tables_imports = module.imported_tables.len();
+    let num_tables_imports = module.local.num_imported_tables;
     let mut table_relocs = Vec::with_capacity(module.local.table_plans.len() - num_tables_imports);
     for (index, table) in module.local.table_plans.iter().skip(num_tables_imports) {
         let def_index = module.local.defined_table_index(index).unwrap();
@@ -66,7 +66,7 @@ pub fn layout_vmcontext(
         });
     }
 
-    let num_globals_imports = module.imported_globals.len();
+    let num_globals_imports = module.local.num_imported_globals;
     for (index, global) in module.local.globals.iter().skip(num_globals_imports) {
         let def_index = module.local.defined_global_index(index).unwrap();
         let offset = ofs.vmctx_vmglobal_definition(def_index) as usize;

crates/obj/src/function.rs

@@ -11,7 +11,7 @@ pub fn declare_functions(
     module: &Module,
     relocations: &Relocations,
 ) -> Result<()> {
-    for i in 0..module.imported_funcs.len() {
+    for i in 0..module.local.num_imported_funcs {
         let string_name = format!("_wasm_function_{}", i);
         obj.declare(string_name, Decl::function_import())?;
     }
@@ -32,7 +32,7 @@ pub fn emit_functions(
 ) -> Result<()> {
     debug_assert!(
         module.start_func.is_none()
-            || module.start_func.unwrap().index() >= module.imported_funcs.len(),
+            || module.start_func.unwrap().index() >= module.local.num_imported_funcs,
         "imported start functions not supported yet"
     );