Boogie

Verification task per split (#841)

Dependent Dafny PR: https://github.com/dafny-lang/dafny/pull/5031

### Changes
- Change the API `executionEngine.GetImplementationTasks` to
`executionEngine.GetVerificationTasks`, which returns a task for each
statically defined assertion batch. When using the split on every assert
option, this will return a verification task for each assertion.

### Testing
- Added test SplitOnEveryAssertion

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Co-authored-by: Aaron Tomb <[email protected]>