Use CFString API instead of ICU in case of iOS

[dinhvh]

To do transliteration and tokenization.
https://developer.apple.com/librarY/mac/documentation/CoreFoundation/Reference/CFStringTokenizerRef/Reference/reference.html#//apple_ref/doc/constant_group/Tokenization_Modifiers
and CFStringTransform()

cc @iljaiwas

[iljaiwas]

There is also an NSString API which supposedly does the same thing:
https://developer.apple.com/library/ios/documentation/cocoa/reference/foundation/classes/NSString_Class/Reference/NSString.html#jumpTo_60

Using CFStringTokenizer is probably faster and can be called from plain C/C++, though.