extern alloc only when 'alloc' feature is enabled

[shamchak808]

I currently use this in a no_std environment. The latest version bump broke my builds with the following error

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait

Looking at this crate, I noticed that there was an alloc feature added. I think a #[cfg(feature = "alloc")] was missing where extern crate alloc was added.

Let me know if I'm missing something, though. Thank you!

[Lymia]

Fixed, and published a new version with this patch.