Add Spanned::new(Range<usize>, T)

[dtolnay]

I looked for why this might not exist and found #368, which is concerned with whether Range<usize> is really the right representation for the field type or if it should hold something else, like a custom Span type defined by serde_spanned.

The same concern might apply to a new constructor, and if so, an approach that doesn't preclude construction with a better type in the future might be preferred. For example Spanned::new(value).with_range(range).

Without some API to construct a Spanned at a given range, I do not know of any way to deserialize an untagged enum containing Spanned fields. Context: dtolnay/serde-untagged#5