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/spec/sequences-spec.janet

@@ -103,3 +103,18 @@
   ["take cycle"             "[1 2 1 2 1]" "(take 5 (cycle [1 2]))"]
   ["take repeatedly"        "3"           "(count (take 3 (repeatedly (fn [] 1))))"]
   ["take-last of range"     "[8 9]"       "(take-last 2 (range 10))"])
+
+# Clojure IFn values used as the function arg to higher-order fns: a keyword or
+# symbol looks up a key, a set tests membership, a map looks up a key.
+(defspec "seq / IFn values as functions"
+  ["map keyword"        "[1 2 3]"        "(map :a [{:a 1} {:a 2} {:a 3}])"]
+  ["filter keyword"     "[{:ok true}]"   "(filter :ok [{:ok true} {:ok false}])"]
+  ["remove keyword"     "[{:ok false}]"  "(remove :ok [{:ok true} {:ok false}])"]
+  ["sort-by keyword"    "[{:a 1} {:a 2} {:a 3}]" "(sort-by :a [{:a 3} {:a 1} {:a 2}])"]
+  ["sort-by key + cmp"  "[{:a 3} {:a 2} {:a 1}]" "(sort-by :a > [{:a 3} {:a 1} {:a 2}])"]
+  ["filter set"         "[2 4]"          "(filter #{2 4} [1 2 3 4 5])"]
+  ["remove set"         "[1 3 5]"        "(remove #{2 4} [1 2 3 4 5])"]
+  ["group-by keyword"   "{1 [{:n 1}], 2 [{:n 2}]}" "(group-by :n [{:n 1} {:n 2}])"]
+  ["map a map"          "[1 nil 2]"      "(map {:a 1 :b 2} [:a :z :b])"]
+  ["take-nth transducer" "[0 2 4 6 8]"   "(into [] (take-nth 2) (range 10))"]
+  ["interpose transducer" "[1 :x 2]"     "(into [] (interpose :x) [1 2])"])