A few small optimizations for CIDFontType2 fonts

Cache a constant length and replace one usage of |in| with a comparison against undefined.
A few small optimizations of adjustMapping

Replace a couple of |in| checks with comparisons against undefined.
[CIDFontType2] Map characters missing in toUnicode to the private use area (bug 1028735 and issue 4881)
Merge pull request #5017 from Snuffleupagus/bug-1028735

[CIDFontType2] Map characters missing in toUnicode to the private use area (bug 1028735 and issue 4881)