bump RAC
++RAC
++RAC
++RAC to libextobjc-subtree
Add the used parts of libextobjc as a subtree
Remove xcworkspace
Add RAC to the project, fix up file references
Stop linking RAC and Archimedes on iOS

Require applications at the top level to do it instead.
Actually, we can remove libextobjc entirely

RAC exports EXTScope, and that's all we need.
Remove Archimedes and RAC imports from prefix header
Add a demo workspace and fix them builds
Update README
Update script/cibuild
++RAC
++RAC
++RAC
++RAC
++RAC
++Archimedes to mac-framework
Link Archimedes.framework