Use extreme assertion for metadata mapped assert (#812)

The `assert_metadata_mapped` method uses the `mmap` system call and
expects a failure. This is too costly even for debug builds. We disable
it unless the "extreme_assertions" feature is enabled.

src/policy/marksweepspace/malloc_ms/metadata.rs

@@ -280,7 +280,7 @@ pub(super) unsafe fn unset_chunk_mark_unsafe(chunk_start: Address) {
 pub(super) unsafe fn load128(metadata_spec: &SideMetadataSpec, data_addr: Address) -> u128 {
     let meta_addr = side_metadata::address_to_meta_address(metadata_spec, data_addr);
 
-    #[cfg(debug_assertions)]
+    #[cfg(all(debug_assertions, feature = "extreme_assertions"))]
     metadata_spec.assert_metadata_mapped(data_addr);
 
     meta_addr.load::<u128>()

src/util/metadata/side_metadata/global.rs

@@ -406,6 +406,7 @@ impl SideMetadataSpec {
         #[cfg(debug_assertions)]
         {
             self.assert_value_type::<T>(input);
+            #[cfg(feature = "extreme_assertions")]
             self.assert_metadata_mapped(data_addr);
         }