core: Stage 2 Task 2 tier 4b — compile ns (+ use/import/refer-clojure)

ns becomes a macro (00-syntax) expanding to (do (in-ns 'name) (require …)
(use …) (import …) (refer-clojure …)) — matching Clojure, where require is
a fn and ns expands to require calls. use/import/refer-clojure join the
namespace ops as ctx-capturing clojure.core fns (use-impl/import-impl/
refer-clojure-impl, interned by install-stateful-fns!). Removed the ns
interpreter special arm; dropped ns/use/import from host_iface special-names
and loader stateful-head?.

Placement matters: ns lives in the FIRST overlay tier (00-syntax), because
the self-hosted analyzer build is triggered while 10-seq loads and processes
jolt.analyzer's own (ns …) form — ns must exist by then. Its body resolves
fn/map/reduce/cond at expansion time, by which point all of 00-syntax has
loaded. The bootstrap compiler (compiler.janet) still PUNTS ns/in-ns/require/
use/import (it compiles analyzer.clj/ir.clj, whose ns forms must fall back to
the interpreter that expands the macro) — only the self-hosted analyzer
compiles them.

use-impl fixes a latent bug: it now refers the used ns's vars into the
CURRENT ns (the old special interned a ns into itself, a no-op).

ns/use/import now compile + interpret as plain invokes. fallback-zero: ns off
must-punt, onto must-compile.

Gate green: conformance 267x3, fallback-zero 38/4, bootstrap-fixpoint
stage1==2==3, self-host, staged-bootstrap, sci-bootstrap, clojure-test-suite
>=4034/67, namespace, deps-loader, cli, aot, embedded-stdlib, uberscript,
features 78/78, all unit + spec.

test/integration/fallback-zero-test.janet

@@ -47,6 +47,7 @@
    # compiles over an INTERNED var; the built-in dynamic vars aren't interned yet,
    # so it's exercised end-to-end in the state spec instead.)
    "(require (quote [clojure.string :as s]))" "(in-ns (quote foo.bar))"
+   "(ns foo.bar (:require [clojure.string :as s]))"
    "(defprotocol P (m [x]))" "(extend-type Long P (m [x] x))"
    "(reify P (m [this] 1))" "(var map)"])
 
@@ -55,7 +56,7 @@
 # (require/in-ns/protocols/binding now compile). The remaining frozen/uncompiled
 # set keeps the harness honest in the punt direction.
 (def must-punt
-  ["(ns foo.bar)" "(defmacro m [x] x)" "(deftype T [a])"
+  ["(defmacro m [x] x)" "(deftype T [a])"
    "(set! *warn-on-reflection* true)" "(letfn [(f [n] (g n)) (g [n] (f n))] (f 1))"])
 
 (var fails @[])