Make IdentityCMaps more compact.

IdentityCMap uses an array to represent a 16-bit unsigned identity
function. This is very space-inefficient, and some files cause multiple
IdentityCMaps to be instantiated (e.g. the one from #4580 has 74).

This patch make the representation implicit.

When loading the PDF from issue #4580, this change reduces peak RSS from
~370 to ~280 MiB. It also improves overall speed on that PDF by ~30%,
going from 522 ms to 366 ms.
Merge pull request #5135 from nnethercote/identity-cmap-proper

Make IdentityCMaps more compact.