improve debug memory-stats options

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/alloc/hardened.rs

@@ -7,9 +7,9 @@ pub fn trim() -> crate::Result { Ok(()) }
 
 #[must_use]
 //TODO: get usage
-pub fn memory_usage() -> Option<string> { None }
+pub fn memory_usage() -> Option<String> { None }
 
 #[must_use]
-pub fn memory_stats() -> Option<String> {
+pub fn memory_stats(_opts: &str) -> Option<String> {
 	Some("Extended statistics are not available from hardened_malloc.".to_owned())
 }