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.
only convert in IE9 if the requested responseType is actually an arraybuffer, otherwise just return the responseText. That way the compatibility.js does not break other frameworks like angular that not always want an array from xhr requests.
Merge pull request #5150 from nnethercote/toUnicode

Fix #4935
Merge pull request #5160 from tomdw/feature/fix-angular-compatibility-ie9-conflict

only convert in IE9 if the requested responseType is actually an arraybu...