Enable marking if commands as {:allow_split} or not

[keyboardDrummer]

Review the commits separately. Refactorings have been placed in the first commit shown in the commits tab.

Changes

• Fix concurrency issue that could create a crash when using multiple cores and simple programs (commit)
• Introduce the attribute {:allow_split} that can be placed on IfCmd and GotoCmd. When using {:isolate "paths"} only, jumps with {:allow_split} will trigger splitting of the VC.

Testing

Updated existing tests so they exercise the presence and absence of {:allow_split}