Think about simplified scoping rules for parallel macros

There seems to be some general confusion amongst the user base for exactly how variable references in an expression passed to one of the parallel macros are resolved and when the value of variables is transferred vs treated as a reference to a global variable on some module defined on the worker.

Part of the problem could be resolved with better documentation, but I also wonder if we can find a set of rules that is more intuitive and robust in the Julia 0.5 timeframe.

Here are some specific issues concerning symbol resolution in parallel macros issues already brought up that we could maybe make some headway on:

• https://github.com/JuliaLang/julia/issues/2669
• https://github.com/JuliaLang/julia/issues/5930
• https://github.com/JuliaLang/julia/issues/6760
• https://github.com/JuliaLang/julia/issues/9118
• https://github.com/JuliaLang/Distributed.jl/issues/19
• https://github.com/JuliaLang/julia/issues/12367
• https://github.com/JuliaLang/Distributed.jl/issues/20
• https://github.com/JuliaLang/julia/issues/11228
• https://github.com/JuliaLang/julia/issues/14399

Start by reviewing the linked Julia issues about symbol resolution in parallel macros, especially the unresolved Distributed.jl issues 19 and 20, and compare them with the checked-off issues listed here. The issue does not name files, tests, or a defined implementation scope; done would require an agreed, more intuitive scoping model and corresponding documentation.