Modernize and fix Objective-C code

[mittsh]

• init methods should return instancetype
• get rid of this ugly _init method – replace by method inheritance
• add nullability info to header files
• add doc strings to methods in header files (especially for delegate methods)
• declare NS_DESIGNATED_INITIALIZER
• replace +CRLFData and +CRLFCRLFData by static FOUNDATION_EXPORT NSData* created on +load