Reshaping/Retyping lenses

[cfhammill]

In the now closed #9 we proposed adding type conversion lenses.

In general we could have type conversion lenses

• [ ] df2list (with suitable arg checking on set)
• [ ] list2df (with suitable arg checking on view)
• [ ] vec2list (with suitable arg checking on set)
• [ ] list2vec (with suitable arg checking on view)
• [ ] vec2vec (perhaps using https://github.com/r-lib/vctrs)

we could also have reshaping lenses

• [ ] flatten_l: turn a nested list into a flat one
• [x] reshape_l: change the dimensions of an array subject to the condition that prod(dim(x)) is preserved.