Made ArrayVec::new and ArrayString::new const fns

Added utils module with a `MaybeUninit` helper type to construct `[MaybeUninit<T>; N]`

Removed all uses of the "unstable-const-fn" feature, and documented it as being deprecated.

Changed `assert_capacity_limit` macro to work in const contexts.

src/utils.rs

@@ -0,0 +1,11 @@
+use std::marker::PhantomData;
+use std::mem::MaybeUninit;
+
+pub(crate) struct MakeMaybeUninit<T, const N: usize>(PhantomData<fn() -> T>);
+
+impl<T, const N: usize> MakeMaybeUninit<T, N> {
+    pub(crate) const VALUE: MaybeUninit<T> = MaybeUninit::uninit();
+
+    pub(crate) const ARRAY: [MaybeUninit<T>; N] = [Self::VALUE; N];
+}
+