Get selection fields from Lookahead

[oeed]

In much the same was as it's useful to get Lookahead from SelectionField #557, the inverse is also quite useful.

This adds a function that generates all SelectionFields fo each field in the Lookahead. I decided to do this over implementing From, but I'm open to changing that.

[sunli829]

Thank you, it would be better if you can add documentation to this function. 😁

[oeed]

Yes good point! Done

[sunli829]

Thank you! 😁