Add a from_array_empty constructor which is a const fn. (#141)

Also lifts the `A: Array` constraint on ArrayVec as it was not used yet
and would otherwise conflict with the const fn, which currently (Rust 1.51)
may not have any trait constraints.

Since this commit only adds a new function and lifts a constraint, this
should be perfectly backwards compatible.

src/arrayvec.rs

@@ -96,10 +96,13 @@ macro_rules! array_vec {
 ///
 /// let more_ints = ArrayVec::from_array_len([5, 6, 7, 8], 2);
 /// assert_eq!(more_ints.len(), 2);
+///
+/// let no_ints: ArrayVec<[u8; 5]> = ArrayVec::from_array_empty([1, 2, 3, 4, 5]);
+/// assert_eq!(no_ints.len(), 0);
 /// ```
 #[repr(C)]
 #[derive(Clone, Copy)]
-pub struct ArrayVec<A: Array> {
+pub struct ArrayVec<A> {
   len: u16,
   pub(crate) data: A,
 }
@@ -954,6 +957,37 @@ impl<A: Array> ArrayVec<A> {
   }
 }
 
+impl<A> ArrayVec<A> {
+  /// Wraps up an array as a new empty `ArrayVec`.
+  ///
+  /// If you want to simply use the full array, use `from` instead.
+  ///
+  /// ## Examples
+  ///
+  /// This method in particular allows to create values for statics:
+  ///
+  /// ```rust
+  /// # use tinyvec::ArrayVec;
+  /// static DATA: ArrayVec<[u8; 5]> = ArrayVec::from_array_empty([0; 5]);
+  /// assert_eq!(DATA.len(), 0);
+  /// ```
+  ///
+  /// But of course it is just an normal empty `ArrayVec`:
+  ///
+  /// ```rust
+  /// # use tinyvec::ArrayVec;
+  /// let mut data = ArrayVec::from_array_empty([1, 2, 3, 4]);
+  /// assert_eq!(&data[..], &[]);
+  /// data.push(42);
+  /// assert_eq!(&data[..], &[42]);
+  /// ```
+  #[inline]
+  #[must_use]
+  pub const fn from_array_empty(data: A) -> Self {
+    Self { data, len: 0 }
+  }
+}
+
 #[cfg(feature = "grab_spare_slice")]
 impl<A: Array> ArrayVec<A> {
   /// Obtain the shared slice of the array _after_ the active memory.