Clojure-compat fixes enabling external HTTP-client support (#159)

Generic language fixes so a host-shim library (jolt-lang/http-client) can carry
clj-http-lite without baking any HTTP/native code into core:

- reader: accept #^ (deprecated metadata reader macro) as ^
- (str pattern) returns the raw regex source, not #"..." — pattern composition
  via (re-pattern (str p ...)) now works
- into/conj onto a map merges map items (was (into {} [{:a 1}]) -> {nil nil})
- a Var is callable as its current value (e.g. (wrap #'f) threading a var client)
- core-class reads a :class field off a thrown table, so libraries can throw
  typed exceptions whose (class e) is a JVM class name
- compiled catch unwraps the interpreter's :jolt/exception envelope (__unwrap-ex),
  so a typed throw keeps its class/message across the interpret/compile boundary
- slurp accepts a byte-array; io/copy is generic over :jolt/input-stream /
  :jolt/output-stream stream markers
- instance? gains a registry (__register-instance-check!) for library shim types
- new clojure.test namespace (deftest/is/are/testing/use-fixtures/run-tests with
  class-aware thrown?/thrown-with-msg?)

Spec: test/spec/clojure-interop-fixes-spec.janet.

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

test/spec/clojure-interop-fixes-spec.janet

@@ -0,0 +1,29 @@
+# Specification: Clojure-compat fixes that landed with HTTP-client support
+# (jolt-lang/http-client). All are general language behaviours, exercised here
+# across interpret + compile modes by the harness.
+(use ../support/harness)
+
+(defspec "interop-fixes / deprecated #^ metadata reader"
+  ["#^ type hint on a param"   "\"x\""
+   "(do (defn f1 [#^String s] s) (f1 \"x\"))"]
+  ["#^\"[B\" array hint"        "[1 2]"
+   "(do (defn f2 [#^\"[B\" b] b) (f2 [1 2]))"]
+  ["#^ is equivalent to ^"      "true"
+   "(= (meta (with-meta [] {:tag (quote String)})) {:tag (quote String)})"])
+
+(defspec "interop-fixes / (str pattern) yields raw source"
+  ["str of a regex"            "\"abc\""        "(str #\"abc\")"]
+  ["compose patterns via str"  "true"
+   "(boolean (re-matches (re-pattern (str #\"<\" \"(.*)\" \">\")) \"<hi>\"))"])
+
+(defspec "interop-fixes / into onto a map"
+  ["merges map items"          "true"   "(= {:a 1 :b 2} (into {} [{:a 1} {:b 2}]))"]
+  ["accepts [k v] pairs"       "true"   "(= {:a 1} (into {} [[:a 1]]))"]
+  ["map item onto empty {}"    "true"   "(= {:x 1} (into {} (list {:x 1})))"]
+  ["conj a map onto {}"        "true"   "(= {:a 1} (conj {} {:a 1}))"])
+
+(defspec "interop-fixes / a var is callable as its value"
+  ["call a var directly"       "42"
+   "(do (def vf (fn [x] (inc x))) ((var vf) 41))"]
+  ["var bound as a client fn"  "\"ok\""
+   "(do (def base (fn [_] \"ok\")) (def mw (fn [client] (fn [req] (client req)))) ((mw (var base)) {}))"])