make use of deprecated namespaces/vars easier to spot

From the mailing list http://groups.google.com/group/clojure/msg/c41d909bd58e4534. It is easy to use deprecated namespaces or vars without knowing you are doing so. The documentation warnings are small, and there is no compiler warning.

Proposed:

• Add new *warn-on-deprecated* dynamic var, defaulted to false
• Warn to stderr when {:deprecated true} namespace is loaded.
• Warn to stderr when {:deprecated true} var is analyzed.
• Warn to stderr when {:deprecated true} macro is expanded.
• New system property clojure.compiler.warn-on-deprecated
• Compile Clojure itself with clojure.compiler.warn-on-deprecated
• Fix deprecation warnings inside Clojure (replicate, clear-agent-errors)
• Mark clojure.parallel as deprecated with :deprecation tag

Examples:

(set! *warn-on-deprecated* true)

;; use of a deprecated var (on compile)
(defn ^:deprecated f [x] x)
(f 5)
;;=> Deprecation warning, NO_SOURCE_PATH:7:1 : var #'user/f is deprecated

;; use of a deprecated macro (on macro expansion)
(defmacro ^:deprecated m [x] x)
(m 5)
;;=> Deprecation warning, NO_SOURCE_PATH:7:1 : macro #'user/m is deprecated

;; use of a deprecated namespace (on load)
(ns foo {:deprecated "1.1"})
(ns bar (:require foo))
;;=> Deprecation warning: loading deprecated namespace `foo` from namespace `bar`

Patch: 706-deprecated-ns-var-warnings-tested-3.diff

Questions: Should default for deprecation warnings be true instead? People upgrading are likely to see new warnings which might be surprising.

• Should default be to warn or not warn on deprecated?