Add ArrayVec::as_inner()

Originally I was going to call this `as_array()`
but `as_inner()` is more consistent with the
existing `into_inner()` method. I don't
_ultimately_ care what it ends up being called.

I wonder if we should have a `impl
From<ArrayVec<T>> for A` implementation or is that
a bit too foot-gun-y?

This lives in its own `impl<A> ArrayVec<A>` block
because rust 1.47.0 is unhappy about having a
const function inside `impl<A: Array>
ArrayVec<A>`. We don't need the trait anyway.

Fixes #193.

src/arrayvec.rs

@@ -1183,6 +1183,18 @@ impl<A: Array> ArrayVec<A> {
   }
 }
 
+impl<A> ArrayVec<A> {
+  /// Returns the reference to the inner array of the `ArrayVec`.
+  ///
+  /// This returns the full array, even if the `ArrayVec` length is currently
+  /// less than that.
+  #[inline(always)]
+  #[must_use]
+  pub const fn as_inner(&self) -> &A {
+    &self.data
+  }
+}
+
 /// Splicing iterator for `ArrayVec`
 /// See [`ArrayVec::splice`](ArrayVec::<A>::splice)
 pub struct ArrayVecSplice<'p, A: Array, I: Iterator<Item = A::Item>> {