Implement java.nio.ByteBuffers backed by unsafe.Ptr

[WojciechMazur]

Resolves #2614

• Adds new java.nio.PointerByteBuffer implementing java.nio.ByteBuffer backed by Ptr[Byte]
• Adds new javalib-intf project used to define extended javalib API (based on Scala.js approach) providing compile time (Classfile) access to Scala Native specific parts of Java Standard Library
• Adds scala.scalanative.memory.PointerBuffer - a user facing API, allowing to wrap Ptr[Byte] and to extract underlying Ptr[_] (to allow for memory management)
• Repeating implementations of multiple buffers and buffer views are now code generated form .gyb template