Fix a couple of bugs

src/alloc.rs

@@ -1,8 +1,9 @@
 use std::{mem, ptr};
 use std::rt::heap;
 use std::sync::atomic::{AtomicUsize, Ordering};
+use std::usize;
 
-const MAX_ALLOC_SIZE: usize = (1 << 32) - 1;
+const MAX_ALLOC_SIZE: usize = usize::MAX;
 
 /// Allocates memory to be used by Bufs or Bytes. Allows allocating memory
 /// using alternate stratgies than the default Rust heap allocator. Also does