Fix & rest destructuring (evaluator.janet): when the original
value is a lazy-seq, derive & rest by walking ls-rest vi steps
instead of slicing the eagerly-realized array from d-realize.
Rewrite core-mapcat as a lazy state machine (core.janet): step
through input colls one element at a time, call f on each tuple,
then yield from f's result collection. No apply-forcing —
all input colls are walked lazily via lazy-from + seq-done?.
Add mapcat to core-renames (compiler.janet) and remove from
Clojure overlay (10-seq.clj) — compile mode now emits direct
calls to the lazy core-mapcat, fixing the pre-existing
protocol-on-record compile error.
Tests: re-enabled mapcat infinite harness case, added 2 & rest
laziness cases. 21/21 lazy-infinite, 229x3 conformance, 32/32 specs.
42da5cef9a