Implement align_up using align_offset from Rust's standard library (#723)

Improve `align_up` performance using a bitmask

src/allocator.rs

@@ -74,11 +74,9 @@ impl<A> Locked<A> {
     }
 }
 
+/// Align the given address `addr` upwards to alignment `align`.
+///
+/// Requires that `align` is a power of two.
 fn align_up(addr: usize, align: usize) -> usize {
-    let remainder = addr % align;
+    (addr + align - 1) & !(align - 1)
-    if remainder == 0 {
-        addr // addr already aligned
-    } else {
-        addr - remainder + align
-    }
 }