Add rc.let custom operator

[ajpallares]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

Iteration replaces the whole scope with the current item, so a predicate inside some or reduce cannot reach a value from the enclosing level, which is what ranking, deduplication and "this item versus the others" all need. rc.rootVar only reaches the root, not intermediate levels.

Description

{"rc.let": [{"name": expression, ...}, body]} evaluates body with each name available to var, and the names survive scope rebinding.

• var reads the active data first, bindings second. A binding therefore cannot mask a field the data has, and any predicate without an rc.let around it resolves exactly as before, including missing, which agrees with var on what exists.
• Declarations all evaluate in the enclosing scope, so bindings cannot see each other and their order carries no meaning. Nest a second rc.let to chain them.
• The declaration object is deliberately never evaluated as a whole, since a single-key object anywhere else is an operator call. Names must be non-empty and free of ..