Update to latest nightly

[gmorenz]

• Certain APIs now expect/return NonNull<Opaque> instead of *mut u8. We immediately cast to/from *mut u8 since NonNull doesn't support some required APIs like offset and it seemed cleanest.
• from_size_align now returns a Result instead of an Option. The error type is an opaque struct that contains no information, so we continue to use the existing error handling that prints some information.
• A few things have been renamed (e.g. Heap to Global), so we use the new names.

Fixes #98

[ghost]

I am an idiot for not checking existing PRs before making my own.

[brendanzab]

Any movement on this @Aatch?

[rozbb]

I apologize. I'm the de facto maintainer of this crate. It took me way to long to see this and I apologize. Version 0.3.13 works on nightly as of Jun 16 2018.

[brendanzab]

No worries! Thanks for looking into it!