Adding size_type and value_type members to Array<T>

[Carnë Draug]

I'm thinking of adding size_type and value_type as member types to
Array for better compatibility with STL.  Like so:

  typedef T value_type;
  typedef octave_idx_type size_type;

The reason for this is twofold: 1) someone more used to STL will be
able to use them, 2) makes it easier to write templates that accept
STL and Array.

I don't think there will be any issue with value_type, the only question
is if we should deprecate element_type.

size_type is trickier because STL concepts that have size_type mention
that it is an unsigned integer which octave_idx_type is not.  Also, it
is kind of odd to call it size_type when there's no size method.  The
closest to size() in Octave is numel() which returns an octave_idx_type.
These seems like a small issue compared to not having size_type at all.

Carnë