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/environ/src/module.rs

@@ -30,16 +30,16 @@ pub struct TableElements {
     pub elements: Box<[FuncIndex]>,
 }
 
-/// An entity to export.
+/// An index of an entity.
 #[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
-pub enum Export {
-    /// Function export.
+pub enum EntityIndex {
+    /// Function index.
     Function(FuncIndex),
-    /// Table export.
+    /// Table index.
     Table(TableIndex),
-    /// Memory export.
+    /// Memory index.
     Memory(MemoryIndex),
-    /// Global export.
+    /// Global index.
     Global(GlobalIndex),
 }
 
@@ -150,21 +150,11 @@ pub struct Module {
     /// function.
     pub local: ModuleLocal,
 
-    /// Names of imported functions, as well as the index of the import that
-    /// performed this import.
-    pub imported_funcs: PrimaryMap<FuncIndex, (String, String, u32)>,
-
-    /// Names of imported tables.
-    pub imported_tables: PrimaryMap<TableIndex, (String, String, u32)>,
-
-    /// Names of imported memories.
-    pub imported_memories: PrimaryMap<MemoryIndex, (String, String, u32)>,
-
-    /// Names of imported globals.
-    pub imported_globals: PrimaryMap<GlobalIndex, (String, String, u32)>,
+    /// All import records, in the order they are declared in the module.
+    pub imports: Vec<(String, String, EntityIndex)>,
 
     /// Exported entities.
-    pub exports: IndexMap<String, Export>,
+    pub exports: IndexMap<String, EntityIndex>,
 
     /// The module "start" function, if present.
     pub start_func: Option<FuncIndex>,
@@ -226,10 +216,7 @@ impl Module {
         Self {
             id: NEXT_ID.fetch_add(1, SeqCst),
             name: None,
-            imported_funcs: PrimaryMap::new(),
-            imported_tables: PrimaryMap::new(),
-            imported_memories: PrimaryMap::new(),
-            imported_globals: PrimaryMap::new(),
+            imports: Vec::new(),
             exports: IndexMap::new(),
             start_func: None,
             table_elements: Vec::new(),
@@ -344,4 +331,9 @@ impl ModuleLocal {
     pub fn is_imported_global(&self, index: GlobalIndex) -> bool {
         index.index() < self.num_imported_globals
     }
+
+    /// Convenience method for looking up the signature of a function.
+    pub fn func_signature(&self, func_index: FuncIndex) -> &ir::Signature {
+        &self.signatures[self.functions[func_index]]
+    }
 }