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.

cranelift/wasm/src/translation_utils.rs

@@ -68,7 +68,7 @@ pub struct ElemIndex(u32);
 entity_impl!(ElemIndex);
 
 /// WebAssembly global.
-#[derive(Debug, Clone, Copy, Hash)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub struct Global {
     /// The type of the value stored in the global.
     pub ty: ir::Type,
@@ -79,7 +79,7 @@ pub struct Global {
 }
 
 /// Globals are initialized via the `const` operators or by referring to another import.
-#[derive(Debug, Clone, Copy, Hash)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub enum GlobalInit {
     /// An `i32.const`.
     I32Const(i32),
@@ -102,7 +102,7 @@ pub enum GlobalInit {
 }
 
 /// WebAssembly table.
-#[derive(Debug, Clone, Copy, Hash)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub struct Table {
     /// The type of data stored in elements of the table.
     pub ty: TableElementType,
@@ -113,7 +113,7 @@ pub struct Table {
 }
 
 /// WebAssembly table element. Can be a function or a scalar type.
-#[derive(Debug, Clone, Copy, Hash)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub enum TableElementType {
     /// A scalar type.
     Val(ir::Type),
@@ -122,7 +122,7 @@ pub enum TableElementType {
 }
 
 /// WebAssembly linear memory.
-#[derive(Debug, Clone, Copy, Hash)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub struct Memory {
     /// The minimum number of pages in the memory.
     pub minimum: u32,