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:

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?