v0.3.0 - Additional `PartialEq`, segfault fix

Added

• Some unit tests for (future) debugging purposes.
• Implementations for PartialEq for StackVec with [T, LEN2], &'a [T] and Vec<T>.

Changed

• From<[T, LEN]> is now implemented for From<T, LEN2> instead to also allow building it from arrays that don't consume the full capacity.
  • Note: not a breaking change because this is strictly more powerful than before.

Fixed

• A double free error when iterating-by-ownership over a StackVec with Drop-elements.