Add ArrayMap/ArraySet (slice-backed ordered map/set) and integrate into ds package

[leaxoy]

Motivation

• Provide a compact, slice-backed ordered map implementation with binary-search lookups and efficient iteration for small-to-medium maps via an ArrayMap backed by a sorted slice.
• Provide an ArraySet built on top of ArrayMap to offer ordered set semantics and common set-algebra operations (Union, Intersection, Difference, SymmetricDifference, etc.).
• Expose the new types and constructors from the top-level ds package and update docs/examples to advertise the new, lightweight ordered collection options.

Description

• Added a new arraymap package implementing ArrayMap with core operations, Entry API (OrInsert, AndModify, Insert, Delete), range/iteration support (IterAsc, IterDesc, RangeAsc, RangeDesc, mutable variants), and helper constructors New, NewOrdered, and NewFromMap (files: arraymap.go, entry.go, iter.go, doc.go).
• Added a new arrayset package implementing ArraySet backed by ArrayMap, including ordering, range iteration, set algebra methods, and convenience constructors (files: arrayset.go, iter.go, doc.go).
• Integrated the new types into the top-level ds package by adding aliases and constructors such as ArrayMap, NewArrayMap, NewOrderedArrayMap, ArraySet, NewArraySet, and NewOrderedArraySet, and reorganized some doc exports in ds.go to include the new structures.
• Updated README.md and package docs to list ArrayMap/ArraySet and added a small example showing usage with cmp.Compare.

Testing

• Ran go test ./... across the module and the test suite completed successfully.
• Added comprehensive unit tests for arraymap (arraymap_test.go) covering ordering, entry API, iteration, range/mutation, cloning/clearing, and constructors, and all tests passed.
• Added unit tests for arrayset (arrayset_test.go) covering basic operations, ordering and ranges, set algebra, and clone semantics, and all tests passed.

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Codex Task