Add ToUnicodeMap class.
Add IdentityToUnicodeMap class.

When loading the PDF from issue #4935, this change reduces peak RSS from
~2400 to ~300 MiB, and improves overall speed by ~81%, from 6336 ms to
1222 ms.
Simplify isIdentityUnicode detection.
Merge pull request #5150 from nnethercote/toUnicode

Fix #4935