3031 - better integer overflow protection

This improves on commit 3026; it turns out you need to manually handle
the traps generated by -ftrapv.
  https://gist.github.com/mastbaum/1004768

Signal handling is based on https://spin.atomicobject.com/2013/01/13/exceptions-stack-traces-c.

Various combinations of platform+compiler seem to work very differently:

  gcc everywhere seems to have extremely threadbare ftrapv support

  Clang + OSX generates SIGILL.

  Clang + Linux is advertised to generate SIGABRT, so I handle that out
  of an excess of caution. However, in my experience it seems to kill
  the program (sometimes segfaulting) even without any signal handlers
  installed.

In the process, I realized that all my current builds are using Clang,
so I added one little test on CI to use g++ in commit 3029.

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