Add a way to show where top level eval is in progress

[IanButterworth]

It would be helpful to be able to see where top level eval is at during a running script or test suite, for debugging slow execution/hangs. Especially on remote machines that are harder to send a SIGUSR1/SIGINFO to.

This adds --show-eval=[yes|no|loc]. yes is the full code with location info, loc is just the main expr location which should be helpful-enough.
For example --show-eval=loc (which also is what --show-eval with no args enables)

julia> Pkg.test("Random", julia_args=`--show-eval=loc`)

With this implementation information is limited when test code is within @testsets or scripts which are mostly a big function that is called at the end. Perhaps more can be done to give info in those cases?

Also maybe this should be --trace-eval to go alongside --trace-compile and --trace-dispatch.