Test "option (apple_swift_prefix)", fix it.

There's a bug that extension fields were not correctly handled
in nested submessages (the extension set data was not always
passed down to generated subdecoders).

API Break: Use `Data` type from Foundation for binary data

This changes the current API and will break existing clients.
The protobuf binary serialization methods have changed to use Data:
    ProtobufMessage.init(protobuf:) now takes Data instead of [UInt8]
    ProtobufMessage.serializeProtobuf() now returns Data instead of [UInt8]

The [UInt8] support is still available under a different name:
    ProtobufMessage.init(protobufBytes: [UInt8])
    ProtobufMessage.serializeProtobufBytes() throws -> [UInt8]

Similarly, proto `bytes` fields that were previously rendered
as [UInt8] properties in Swift are now rendered to Data properties.

This change was made to better align Swift Protobuf conventions
with Swift 3 Foundation.

Initial commit of version 0.9.13