core::fmt::Formatter needs setter methods

[Lokathor]

If you're given a format string when formatting a struct there's no way to

1. read some of that data
2. decide how to format the struct fields.
3. have those fields then format themselves according to an alternate Formatter based on your decision above.

Example: if you get a width argument, you can't use some of the width space yourself and then tell an inner field to format itself with a smaller width value because you used some of it.

So Formatter should get methods to change its configuration for things that are safe to change (width, alternate flag, precision, etc)

[Mark-Simulacrum]

Closing in favor of #19207.