feat(core): seqable hash-maps and sets, sequential destructuring

[jig]

What

Hash-maps and sets are now seqable, as in Clojure, and binding forms gain sequential destructuring:

(into {} (map (fn [[k v]] [k (- v)]) {:seconds 5 :days 2}))
;;=> {:seconds -5 :days -2}

• A hash-map seqs as a sequence of [key value] entry vectors, a set as a sequence of its elements — implemented at a single choke point, types.GetSlice (plus ConvertFrom for vec and a seq case). seq, first, rest, map, filter, reduce, apply, cons, concat, vec, into and lazy-seqs work over both.
• Keys/elements are returned exactly as stored (what keys already does), in sorted order. Sorting is a correctness requirement, not cosmetics: first and rest each seq the collection independently, and Go randomises map iteration per call, so an unstable order makes a first/rest traversal (lisp-level reduce/filter) drop or repeat entries.
• nth explicitly rejects hash-maps and sets: they seq, but are not indexed collections (Clojure errors here too), and returning a sorted-order element would invite position-dependent code.
• Sequential destructuring in fn params and let bindings ((fn [[k v]] …), (let [[a [b c]] x] …)): positional and recursive, missing elements bind nil, extra are ignored, & binds the remainder as a list. The language previously only had & varargs, so the entry idiom above was impossible. Top-level fn arity checking stays strict; loop/recur bindings remain symbol-only for now.

Behaviour change

⚠️ (seq #{}) now returns nil (Clojure parity) instead of (); (seq {}) is also nil. Two step-test assertions updated accordingly.

Notes

• All GetSlice callers were audited (see ROADMAP-keywords.md §2, added in the first commit): the rest either gain map support with Clojure semantics or are unaffected.
• Full test suite green; BenchmarkMAL1 unchanged within noise.
• New tests in seqable_hashmap_test.go compare multi-entry results order-insensitively.

🤖 Generated with Claude Code