Update to nightly 2021-05-16

This removes the 'const_fn' feature.

Author: Techcable

libs/context/src/lib.rs

@@ -1,7 +1,6 @@
 #![feature(
     negative_impls, // !Send is much cleaner than `PhantomData<Rc>`
     untagged_unions, // I want to avoid ManuallyDrop in unions
-    const_fn, // Apparently this feature is unstable???
     const_fn_trait_bound, // So generics + const fn are unstable, huh?
 )]
 #![cfg_attr(not(feature = "std"), no_std)]

libs/simple/src/lib.rs

@@ -5,7 +5,6 @@
     negative_impls, // impl !Send is much cleaner than PhantomData<Rc<()>>
     exhaustive_patterns, // Allow exhaustive matching against never
     const_alloc_layout, // Used for StaticType
-    const_fn, // We statically create type info
     const_panic, // Const panic should be stable
     untagged_unions, // Why isn't this stable?
     new_uninit, // Until Rust has const generics, this is how we init arrays..