Introduce the Repeat Apalache operator

[Kukovec]

• Tests added for any new code
• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Documentation added for any new functionality
• Entries added to ./unreleased/ for any new functionality

Extends the suite of Apalache operators, by adding Repeat, a repeated iteration operator. Semantically,

Repeat(Op, N, x) = ApaFoldSeqLeft(Op, x, MkSeq(N, LAMBDA i: i))

However, in the encoding, we can avoid constructing the 1 to N sequence explicitly, for an efficiency gain.

[konnov]

Looks good! Please add a few comments (see below). I believe that if we had a separate rewriting framework, we probably could just do that purely by rewriting.