Match core's definition of uninit_array

Since we specifically bound on `ArrayType = [T; N]` it's possible to
write that directly.

Using the exact same code as core should help ensure and inspire
confidence that it's implemented correctly.

Note: a TODO still suggests using the upstream `core` method when it's
been stabilized

src/lib.rs

@@ -225,7 +225,7 @@ where
         // See how `core` defines `MaybeUninit::uninit_array` for a similar example:
         // <https://github.com/rust-lang/rust/blob/917f654/library/core/src/mem/maybe_uninit.rs#L350-L352>
         // TODO(tarcieri): use `MaybeUninit::uninit_array` when stable
-        Self(unsafe { MaybeUninit::<U::ArrayType<MaybeUninit<T>>>::uninit().assume_init() })
+        Self(unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() })
     }
 
     /// Extract the values from an array of `MaybeUninit` containers.