Native record representation + inline nil?/some? (#222)

simon/jolt

Records were a jrec holding an alist of (kw . val) conses: ~113B/node, built
fresh per construction, field reads a list scan. Replace that with a shared
per-type descriptor (tag + field keywords + an eq?-keyed keyword->index table)
plus a flat per-instance value vector and an extension map for any non-field
keys assoc'd on (jolt-nil when there are none). Construction now allocates one
vector instead of a cons chain and a field read is an index lookup. binary-trees
construction allocation drops 2.085GB -> 1.19GB.

That alone barely moved binary-trees wall-time: profiling showed the read loop,
not allocation, dominates, and the read loop's own allocation came from (nil? l)
lowering to (jolt-invoke (var-deref "clojure.core" "nil?") l), which conses its
args every call. Add nil?/some? to the backend native-op table so they inline to
jolt-nil?/jolt-some? (and drop the truthy wrapper, like the other predicates).
check-tree's read loop goes from 1.476GB allocated to zero; binary-trees 18.9x
-> 9.7x vs JVM. The remaining gap is the field-read dispatch chain (jolt-c3mw).

Two JVM divergences fixed along the way, both certified:
- dissoc of a declared field downgrades a record to a plain map (was kept as a
  record); an extension key still drops cleanly.
- map->R keeps extension keys (was dropping anything outside the declared basis).

16 new corpus rows pin assoc/dissoc/count/keys/seq/=/hash/extension-field
behavior against JVM Clojure.

Co-authored-by: Yogthos <yogthos@gmail.com>

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