Remove the const_fn feature.

It has been removed in recent nightlies :)

Author: Techcable

lib/derive/tests/simple.rs

@@ -1,7 +1,7 @@
 #![feature(
     never_type,
-    // const_fn is used by AsmType
-    const_fn, const_panic
+    // RFC has been accepted
+    const_panic
 )]
 
 use static_reflect::{field_offset, StaticReflect, FieldReflect};

src/lib.rs

@@ -11,7 +11,6 @@
 //! of compile-time reflection.
 #![deny(missing_docs)]
 #![feature(
-    const_fn, // We rely on const eval :(
     const_panic, const_option, // We use Option::unwrap
     const_fn_fn_ptr_basics, // We use PhantomData<fn() -> T>
     // Used for field_offset macro