Kill Boogie and Z3 processes when Carbon is interrupted

[dewert99]

Makes sure Carbon kills its Boogie and Z3 subprocesses when interrupted.

[gauravpartha]

Rebased branch onto master

[fpoli]

In case it's useful, Silicon additionally uses a shutdown hook: [1] [2]

[gauravpartha]

In case it's useful, Silicon additionally uses a shutdown hook: [1] [2]

Thanks for the pointer! I have now added an analogous hook to Carbon in this PR.

[marcoeilers]

LGTM