fix #27546 by using jl_set_global instead of eval to set Main.ans

[jrevels]

Thanks to @JeffBezanson for pointing out the problem/this workaround.

Still needs a test.

[jrevels]

I added a test, but I don't really like it - seems a bit fragile and dependent on somewhat irrelevant things (like how we pretty-print Exprs).

If I was less noobish with the REPL code, I'd try to actually test the resultant object/check that no errors are generated without depending on checking pretty-printed results.

[jrevels]

CircleCI failure is unrelated (it's the Could not locate challenge: "Private key location for '[email protected]':"... thing that's been popping up in other PRs as well).

I'm also pretty sure the Travis macOS failure is unrelated, unless our FileWatching for some reason relied on the REPL 😛Error is:

FileWatching: Test Failed at /private/tmp/julia/share/julia/stdlib/v0.7/FileWatching/test/runtests.jl:401
  Expression: pop!(changes) == (F_PATH => FileWatching.FileEvent(FileWatching.UV_RENAME))
   Evaluated: "afile.txt3" => FileWatching.FileEvent(true, false, false) == "afile.txt" => FileWatching.FileEvent(true, false, false)

With a happy green checkmark from Jeff, and CI mostly green, I'll go ahead and merge.