Implement @isdefined

[jekbradbury]

isdefined(::Symbol) was deprecated to the newly-implemented @isdefined in 0.7 as a part of JuliaLang/julia#22064. PR coming.

[yuyichao]

If you do, make sure you read through #402. If you haven't it most likely that your implementation is wrong.

[jekbradbury]

Welp, missed this #402. Closing in favor of trying to figure out what's going on there.