Improve and move operations on sets for Issue 969

[MollyDream]

• We added a section to define numeric types, which is later referred to when defining the argument types of sets and indexes.
• We clarified the implicit casts of both set types and set operations.
• We added examples of implicit casts for set operations in implicit casts.
• We moved the section of Operations on sets before the section of Casts.
• We adapted an existing example to generate a set of int<W>.

Fixes #969

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[MollyDream]

When merging, the description of bit-slicing should be shortened by referring to the "numeric types".

[MollyDream]

As @QinshiWang suggests, maybe we want to rename the "numeric types" to "integral types".

[QinshiWang]

This PR contains two parts, (1) adding the concept of numeric types and cleaning the spec, and (2) adding explanations of set<T>. If not much labor is needed to separate them, we should do so.

[jnfoster]

Subsumed by #1244.