Connections serialization

Every connection is serialized in a single block in connections part.

Example:

- from: source operation name
  to: destination operation name
  from_output: 0       # output number of the source operation, defaults to 0
  to_input: 0          # input number of the destination operation, defaults to 0
  shuffle: 1           # shuffling step, defaults to 1

Shuffling step description is given here.