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
• reshape_l: change the dimensions of an array subject to the condition that prod(dim(x)) is preserved.

[cfhammill]

for type conversion it seems like the general principle is when lensing from a more restricted to a less restricted type you need to be careful when you set. When lensing from a less restricted to a more restricted type you need to be careful when you view. Neat symmetry.