Spec index operator []

In spec expressions, the index operator is treated differently than in exec expressions, where it corresponds to the usual Rust index operator.

Specifically, in a spec expression, the expression expr[i] is a shorthand for expr.spec_index(i). This is a purely syntactic transformation, and there is no particular trait.

For example:

• spec_index for a Seq
• spec_index for a Map
• spec_index for a slice