Improve structural preservation

[twpayne]

A function like toLower lowercases a string, returning a string. Thanks to eachString it can also operate on a []string and return a []string.

This sort of structure-aware mapping (I think the functional programming folk call it fmap would be great to extend to a wider range of types, for example nested slices ([][]string) or maps (map[any]string).

A general solution to this will likely involve some advanced use of reflect and or some gnarly generics work.