MLBEDSW-10299: Size types and allocator fixes for ordered_map

 - Normalise the sized types used by ordered_map to present
   an interface more consistent with the standard library.
 - Set initial allocation to zero such that declaring an
   empty map allocates no storage, and add tests for the
   same. Storage is now allocated on first-use.
 - Fix potential range issue with initial hashtable size
   being greater than the chosen indexer.
 - Fix issue where it was not possible to resize up to
   the maximum indexer limit.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: I12742431808d73625ac6bcdbd7b701b52f763834