Replace elastic_integer, introduce families

[johnmcfarlane]

What would you like to see in CNL?

As per feedback in Cologne, replace elastic_integer with two types:

1. A promotion_integer type which auto-widens, taking a family as its only parameter.
2. A digit_integer type which has Digits and a family as its parameters.

This leaves room for a width_integer, which potentially performs more overflow checks (when composed with overflow_integer) but permits the MNN, thus increasing the range of the type by 1. Its parameters are Width and family. It is advised against for nearly all use cases.

What are you trying to do?

Unblock progress toward standardisation and resolve the confusion surrounding the purpose of elastic_integer in #851.