fix: value-semantics for collection keys/elements; set literals evaluate

Close jolt-do7. Maps/sets keyed by a collection (a map, vector, ...) now compare
by value instead of Janet identity:
- PHM/PHS hash and compare keys through an injected canonicalizer (collection
  keys -> value-hashable struct/tuple); keys are still stored as-is
- map literals and core-assoc promote to a phm when a key is a collection
- frequencies/group-by use a phm base so collection elements/keys dedup by value
- set equality is value-based (from earlier)

Real bugs found and fixed along the way:
- set literals #{(inc 1)} did not evaluate their elements (stored raw forms!)
- the REPL printer rendered phm maps as {} (they hit the deftype branch); now a
  phm branch prints entries

Added spec cases (maps/collection-keys, sets/literals & value elements).
conformance 218/218, jpm test green.

test/spec/sets-spec.janet

@@ -23,6 +23,13 @@
   ["set as fn present"      "2"         "(#{1 2 3} 2)"]
   ["set as fn missing"      "nil"       "(#{1 2 3} 9)"])
 
+(defspec "set / literals & value elements"
+  ["literal evaluates elements" "#{2 4}" "#{(inc 1) (* 2 2)}"]
+  ["map elements by value"  "true"      "(= #{{:a 1}} #{(hash-map :a 1)})"]
+  ["contains? map by value" "true"      "(contains? #{(hash-map :x 1)} {:x 1})"]
+  ["dedup equal maps"       "1"         "(count (set [{:a 1} (hash-map :a 1)]))"]
+  ["vector elements"        "true"      "(contains? #{[1 2]} (vec [1 2]))"])
+
 (defspec "clojure.set"
   ["union"                  "#{1 2 3 4}" "(do (require (quote [clojure.set :as s])) (s/union #{1 2} #{3 4}))"]
   ["intersection"           "#{2}"       "(do (require (quote [clojure.set :as s])) (s/intersection #{1 2} #{2 3}))"]