Field should become a type instead of an interface

[nhooyr]

Type's should not be able to define their default logging key. I can easily see this causing bugs and would prefer the consistent explicit style of always writing out the key. If its getting duplicated a lot, With is probably needed instead.

[nhooyr]

In fact, we could just turn Field into type struct F { Name string; Value interface{} } and also remove slog.F as is, reducing the API surface area further. This would go nicely with #47 as fields would be predominately specified with { now instead of (.

[nhooyr]

@coadler how do you feel about this?

[nhooyr]

This is an example of how logging would look:

	slogtest.Info(t, "my message here",
		slog.F{"field_name", "something or the other"},
		slog.F{"field_name", "something or the other"},
		slog.F{"some_map", slog.Map{
			{"nested_fields", "wowow"},
		}},
		slog.Error(xerrors.Errorf("wrap1: %w",
			xerrors.Errorf("wrap2: %w",
				io.EOF),
		)),
	)