Enable marking if commands as {:allow_split} or not

[keyboardDrummer]

Depends on #968

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.
• Make it so that when using {:vcs_split_on_every_assert}, no additional VCs are introduced for path that do not end in an explicit return. In additional, VCs introduced for explicit returns will only contain a single assertions, even when there are multiple ensures clauses.

Testing

• Updated existing tests so they exercise the presence and absence of {:allow_split}
• Added a test that combines {:vcs_split_on_every_assert} and explicit returns.