Alloc a slice of bytes with the given size. Sizes till MaxSize are supported, for larger
sizes nil will be returned. The slices are taken from sync.Pool caches sized to the
nearest power of two and truncated to the size supplied. This function may be
called concurrently.
Free a byte slice allocated by Alloc. The argument may be shorted from the original allocation,
it is reset to len = cap. This function may be called concurrently.