feat(c-api) Implement Drop for wasm_exporttype_t.

`wasm_exporttype_t` has 2 fields: `name` and `extern_type`. Both are
of kind `NonNull`. When `wasm_exporttype_t` is dropped, nor `name` nor
`extern_type` are going to be dropped.

To avoid leaking data, this patch adds a new field: `owns_fields`:

* When `wasm_exporttype_t` is built from `wasm_exportype_new`, this
  field is set to `false` because `name` and `extern_type` are
  received by pointer, and its the responsibility of the caller to
  free them,

* When `wasm_exporttype_t` is built from the `From<&ExportType>`
  implementation, _we_ create `name` and `extern_type` to then leak
  them. In this case, it is safe to reconstruct proper `Box`es to
  finally drop them.

lib/c-api/src/wasm_c_api/types/export.rs

@@ -6,6 +6,7 @@ use wasmer::ExportType;
 pub struct wasm_exporttype_t {
     name: NonNull<wasm_name_t>,
     extern_type: NonNull<wasm_externtype_t>,
+    owns_fields: bool,
 }
 
 wasm_declare_boxed_vec!(exporttype);
@@ -15,7 +16,11 @@ pub extern "C" fn wasm_exporttype_new(
     name: NonNull<wasm_name_t>,
     extern_type: NonNull<wasm_externtype_t>,
 ) -> Box<wasm_exporttype_t> {
-    Box::new(wasm_exporttype_t { name, extern_type })
+    Box::new(wasm_exporttype_t {
+        name,
+        extern_type,
+        owns_fields: false,
+    })
 }
 
 #[no_mangle]
@@ -33,6 +38,20 @@ pub extern "C" fn wasm_exporttype_type(
 #[no_mangle]
 pub extern "C" fn wasm_exporttype_delete(_exporttype: Option<Box<wasm_exporttype_t>>) {}
 
+impl Drop for wasm_exporttype_t {
+    fn drop(&mut self) {
+        if self.owns_fields {
+            // SAFETY: `owns_fields` is set to `true` only in
+            // `wasm_exporttype_t.from(&ExportType)`, where the data
+            // are leaked properly and won't be freed somewhere else.
+            unsafe {
+                let _ = Box::from_raw(self.name.as_ptr());
+                let _ = Box::from_raw(self.extern_type.as_ptr());
+            }
+        }
+    }
+}
+
 impl From<ExportType> for wasm_exporttype_t {
     fn from(other: ExportType) -> Self {
         (&other).into()
@@ -59,6 +78,10 @@ impl From<&ExportType> for wasm_exporttype_t {
             unsafe { NonNull::new_unchecked(Box::into_raw(Box::new(extern_type))) }
         };
 
-        wasm_exporttype_t { name, extern_type }
+        wasm_exporttype_t {
+            name,
+            extern_type,
+            owns_fields: true,
+        }
     }
 }