fix: keyword/set/map as IFn in higher-order fns; nil-seq; take-nth/interpose/sort-by/min-key arities

Biggest fix: jolt keywords are Janet keywords and maps are Janet structs/phm, but
(:a struct) at the Janet level returns nil (not a Clojure accessor) and errors on
a phm — so core-map/filter/sort-by/group-by/etc. calling (f x) directly broke the
ubiquitous keyword/set/map-as-function idioms ((map :a coll), (sort-by :k coll),
(filter a-set coll), (group-by :type coll)). Added as-fn coercion (keyword/symbol
-> key lookup, map -> key lookup, set -> membership) applied at the entry of
map/filter/remove/keep/mapv/filterv/sort-by/group-by/partition-by/some/
not-any?/not-every?/take-while/drop-while/min-key/max-key.

Also:
- realize-for-iteration treats nil as an empty seq (Clojure semantics), fixing
  nth/nthrest/nthnext/take-last/reduce/doseq over nil.
- take-nth and interpose gained their 1-arg transducer arities.
- sort-by gained the 3-arg (keyfn comparator coll) form.
- min-key/max-key: single item returns it without calling f; ties keep the last.
- underive gained the 2-arg global-hierarchy form.

clojure-test-suite pass 3535->3649, errors 177->122, clean files 39->44.
spec: seq/IFn-values-as-functions (11). jpm test green.

test/integration/clojure-test-suite-test.janet

@@ -18,9 +18,9 @@
 
 # Baseline: assertions Jolt currently passes across the suite. Raise as Jolt
 # improves so a regression (previously-passing assertion breaking) is caught.
-(def baseline-pass 3450)
+(def baseline-pass 3600)
 # A file is "clean" when it ran with zero failures AND zero errors.
-(def baseline-clean-files 38)
+(def baseline-clean-files 43)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;
 # this only fires on infinite-sequence hangs.
 (def per-file-timeout 6)