Hlo parser: support fusion.

Also,
- Add a HloInstruction::CreateFusion interface that creates a fusion instruction with given fusion computation. Add a HloComputation::SetFusionInstruction interface to help do that.
- Change how we print fusion kind. Before this change we print fusion kind together with the opcode, e.g., fusion:kLoop, which is not easy to parse. Now we append fusion kind as an attribute.
- Print fusion computation the same way as other computations, instead of nested in an instruction.

PiperOrigin-RevId: 175621768