NSData: Align methods with Darwin

- Remove 'convenience' from:
  init()
  init(bytes:length:)
  init(bytesNoCopy:length:)
  init(bytesNoCopy:length:freeWhenDone:)
  init(bytesNoCopy:length:deallocator:)
  init(contentsOfFile:options:)
  init?(contentsOfFile:)
  init(data:)
  init(contentsOf:,options:)
  init?(base64Encoded base64String:options:)
  init?(base64Encoded base64Data:options:)

- Add:
  init?(contentsOf:)
  (deprecated) base64Encoding()

- Add (unimplemented):
  (deprecated) init?(contentsOfMappedFile:)
  (deprecated) class func dataWithContentsOfMappedFile()
  contains()
  contains(where:)

NSMutableData:

- Remove 'convenience' from:
   init?(capacity:)
   init?(length:)

- Add:
  init(bytes:length:)

- Add description documentation to public methods.

- Update tests.

Author: spevans