Remove the need for HostRef<Store> (#771)

* Remove the need for `HostRef<Store>`

This commit goes through the public API of the `wasmtime` crate and
removes the need for `HostRef<Store>`, as discussed in #708. This commit
is accompanied with a few changes:

* The `Store` type now also implements `Default`, creating a new
  `Engine` with default settings and returning that.

* The `Store` type now implements `Clone`, and is documented as being a
  "cheap clone" aka being reference counted. As before there is no
  supported way to create a deep clone of a `Store`.

* All APIs take/return `&Store` or `Store` instead of `HostRef<Store>`,
  and `HostRef<T>` is left as purely a detail of the C API.

* The `global_exports` function is tagged as `#[doc(hidden)]` for now
  while we await its removal.

* The `Store` type is not yet `Send` nor `Sync` due to the usage of
  `global_exports`, but it is intended to become so eventually.

* Touch up comments on some examples

* Run rustfmt

crates/api/src/wasm.rs

@@ -609,7 +609,7 @@ pub unsafe extern "C" fn wasm_func_new(
     ty: *const wasm_functype_t,
     callback: wasm_func_callback_t,
 ) -> *mut wasm_func_t {
-    let store = &(*store).store;
+    let store = &(*store).store.borrow();
     let ty = (*ty).functype.clone();
     let callback = Rc::new(callback);
     let func = Box::new(wasm_func_t {
@@ -663,7 +663,7 @@ pub unsafe extern "C" fn wasm_instance_new(
     imports: *const *const wasm_extern_t,
     result: *mut *mut wasm_trap_t,
 ) -> *mut wasm_instance_t {
-    let store = &(*store).store;
+    let store = &(*store).store.borrow();
     let mut externs: Vec<Extern> = Vec::with_capacity((*module).imports.len());
     for i in 0..(*module).imports.len() {
         let import = *imports.add(i);
@@ -731,7 +731,7 @@ pub unsafe extern "C" fn wasm_module_new(
     binary: *const wasm_byte_vec_t,
 ) -> *mut wasm_module_t {
     let binary = (*binary).as_slice();
-    let store = &(*store).store;
+    let store = &(*store).store.borrow();
     let module = Module::new_unchecked(store, binary).expect("module");
     let imports = module
         .imports()
@@ -765,7 +765,7 @@ pub unsafe extern "C" fn wasm_module_validate(
     binary: *const wasm_byte_vec_t,
 ) -> bool {
     let binary = (*binary).as_slice();
-    let store = &(*store).store;
+    let store = &(*store).store.borrow();
     Module::validate(store, binary).is_ok()
 }
 
@@ -814,7 +814,7 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
     env: *mut std::ffi::c_void,
     finalizer: std::option::Option<unsafe extern "C" fn(arg1: *mut std::ffi::c_void)>,
 ) -> *mut wasm_func_t {
-    let store = &(*store).store;
+    let store = &(*store).store.borrow();
     let ty = (*ty).functype.clone();
     let callback = Rc::new(CallbackWithEnv {
         callback,
@@ -1337,7 +1337,7 @@ pub unsafe extern "C" fn wasm_global_new(
     val: *const wasm_val_t,
 ) -> *mut wasm_global_t {
     let global = HostRef::new(Global::new(
-        &(*store).store,
+        &(*store).store.borrow(),
         (*gt).globaltype.clone(),
         (*val).val(),
     ));
@@ -1456,7 +1456,10 @@ pub unsafe extern "C" fn wasm_memory_new(
     store: *mut wasm_store_t,
     mt: *const wasm_memorytype_t,
 ) -> *mut wasm_memory_t {
-    let memory = HostRef::new(Memory::new(&(*store).store, (*mt).memorytype.clone()));
+    let memory = HostRef::new(Memory::new(
+        &(*store).store.borrow(),
+        (*mt).memorytype.clone(),
+    ));
     let m = Box::new(wasm_memory_t { memory, ext: None });
     Box::into_raw(m)
 }
@@ -1549,7 +1552,11 @@ pub unsafe extern "C" fn wasm_table_new(
         Val::AnyRef(AnyRef::Null)
     };
     let t = Box::new(wasm_table_t {
-        table: HostRef::new(Table::new(&(*store).store, (*tt).tabletype.clone(), init)),
+        table: HostRef::new(Table::new(
+            &(*store).store.borrow(),
+            (*tt).tabletype.clone(),
+            init,
+        )),
         ext: None,
     });
     Box::into_raw(t)