F/value range add sub helpers

[maurolacy]

Add a couple of helpers to ValueRange for non-transactional addition and subtraction.

Names clash sometimes (but not always) with the std::ops::Add impl. That's unfortunate, but I couldn't think of better names. In any case, clashing can be avoided by using the fully qualified syntax for addition. (solved it by replacing the Add impl by a helper function)

Also adds a stock Display impl.

[jawoznia]

Not necessary in this PR but we could change prepare_add to something like reserve_add. This keyword well describes the purpose and is also used in the comment.

Also should this be merged to the main?

[maurolacy]

Not necessary in this PR but we could change prepare_add to something like reserve_add. This keyword well describes the purpose and is also used in the comment.

I would stick to prepare_add for now.