This reverts commit 09093a2.

This reverts commit 09093a2.

### Changes
- Undo revert #966, which was
reverted because it caused unintended Dafny changes
- Change the isolation of returns, so that each ensures clause gets its
own VC when `{:vcs_split_on_every_assert}` is used, so we get #returns *
#ensures additional VCs from return statements.
- Change the isolation of returns, so that only explicit return
statement are given their own VC branch

### Testing
- Added a CLI test that tests the above two changes
- I have checked that updating Dafny to this latest Boogie version only
causes expected failures in Dafny tests that can easily be repaired.