Buffers: getRemainingBytes(Object) -> remainingBytes(Object); sizeOfBufferElem(Buffer) -> sizeOfBufferElem(Object) to include NativeBuffer<?>

Misc:
  - Add remainingElem(Object buffer).
  - Removed 'sizeOfBufferType(Class<?> bufferType)', since we don't use such calling convention w/ class type

Note: remainingBytes(..) exist to allow using only one branch traversal to return the remaining size in bytes
      instead of 2, remaining(obj) and sizeOfBufferElem(obj).
Note: The methods can take NativeBuffer<?> as an argument.