Define scope macros with conditional @try/@autoreleasepool

Back the scope macros with @autoreleasepool for DEBUG builds, and @try
otherwise. This prevents suppressed return warnings in DEBUG, and otherwise
will prevent unnecessary autorelease pool allocations.

See jspahrsummers/libextobjc#51
Namespace `keywordify` with ext_
Merge remote-tracking branch 'origin/master' into autoreleasepool-workaround
Comment on ext_keywordify
Better word
Merge pull request #68 from kastiglione/autoreleasepool-workaround

Scope macro keyword workaround