handle the case where 0 or >1 allocs are enabled

In particular this fixes `cargo build --all-features`.

src/alloc/default.rs

@@ -0,0 +1,7 @@
+//! Default allocator with no special features
+
+/// Always returns the empty string
+pub(crate) fn memory_stats() -> String { Default::default() }
+
+/// Always returns the empty string
+pub(crate) fn memory_usage() -> String { Default::default() }