Support stable Rust

[hannobraun]

This crate currently requires a nightly Rust toolchain. We currently use the following experimental features:

• const_fn (documentation, tracking issue)
• const_refcell_new (documentation, tracking issue)
• macro_reexport (documentation, tracking issue)
• never_type (documentation, tracking issue)
• use_extern_macros (documentation, tracking issue)

It think supporting stable Rust would be nice, but I don't see this as a pressing issue. It's probably best to just wait for the various experimental features to stabilize and re-evaluate then.

[hannobraun]

Updated the original comment. macro_reexport has been replaced with use_extern_macros.

[hannobraun]

Updated original comment. As of #85, const_refcell_new and use_extern_macros are no longer used and all dependencies build on beta.

[hannobraun]

Parts of const fn have been stabilized: rust-lang/rust#54835

Need to reevaluate this issue.