Construct operator tags with state

[rkaminsk]

Optionally, support constructing operator tags from the parse state and an input iterator.

Notes

• I did not know how to add a state into the verify construction of the exiting operator tests so I simply added tests separately.
• Maybe you have a better idea how to test this? It would be quite hard for me to dive into the details here.
• See also location tracking for operators #170.

[foonathan]

I've merged it with changes in 53b61f0. Thanks.

[rkaminsk]

Thanks, to you. 👍