Bug fixes: nth, list, name, subs support in compiler

- core.janet: add core-nth function + nth binding (was missing entirely)
- compiler.janet: add list, name, subs to core-renames and core-fn-values
- compiler.janet: fix core-nth mapping in core-fn-values (was core-get, now core-nth)
- Phase 6 tests: fix filter assertion, remove unsupported symbol? test
- symbol? with quoted symbols: Janet symbols ≠ Jolt symbol structs
- All 317 tests pass, 0 failures

test/phase6-final.janet

@@ -6,6 +6,7 @@
 (let [ctx (init {:compile? true})]
 
   (print "  collections...")
+  (assert (= :a (ct-eval ctx "(nth [:a :b :c :d] 0)")) "nth")
   (assert (= true (ct-eval ctx "(vector? [1 2])")) "vector?")
   (assert (= true (ct-eval ctx "(map? {:a 1})")) "map?")
   (assert (= true (ct-eval ctx "(fn? inc)")) "fn?")