improve alloc stats interface; fix admin command formatting

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

src/core/alloc/default.rs

@@ -1,9 +1,9 @@
 //! Default allocator with no special features
 
-/// Always returns the empty string
+/// Always returns None
 #[must_use]
-pub fn memory_stats() -> String { String::default() }
+pub fn memory_stats() -> Option<String> { None }
 
-/// Always returns the empty string
+/// Always returns None
 #[must_use]
-pub fn memory_usage() -> String { String::default() }
+pub fn memory_usage() -> Option<String> { None }