Change Termination::report return type to ExitCode
Related to https://github.com/rust-lang/rust/issues/43301
The goal of this change is to minimize the forward compatibility risks in stabilizing Termination. By using the opaque type `ExitCode` instead of an `i32` we leave room for us to evolve the API over time to provide what cross-platform consistency we can / minimize footguns when working with exit codes, where as stabilizing on `i32` would limit what changes we could make in the future in how we represent and construct exit codes.
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