Add `map()` schema function

[kalekundert]

It'd be nice to have some ability to recurse into nested data structures. I think I can already descend into dicts using cast(), but I need another function for containers. Some thoughts:

• I should make an effort to maintain the container type.
• map() might be a good name.
• I might also rename cast() to apply() and defaults() to fill()...
• I could do this using cast(), maybe. Non-keyword argument applied to all items in container (or values if container is dict). But the keyword arguments become limiting. Better to make a new function.