Directly allow numbers as terms

[SReichelt]

It should be possible to reference numbers directly without referring to $number. Instead, as already noted in #46, certain sets should specify how to interpret numbers as elements of that set, by referencing e.g. $number, $inverse, and $quotient (see below).

This would have several advantages:

• It would make files less verbose.
• It would play well with the simplified user interface envisioned in Attach usage suggestions to set definitions #46.
• The "Number tuple" macro would become unnecessary.

Ideally, we want to extend this to integers and rational numbers (including decimal notation); see #46. The type of every number should be determined automatically.

However, we cannot remove the $number macro entirely because when a number appears without context, it is not clear how to interpret it.

Can we somehow extend this to arbitrary (semi)rings? It seems we would need to attach the information to $Carrier then.