Add @compat for standard library imports

[TotalVerb]

Alternative to #404.

See JuliaLang/julia#23931.

The SharedArrays module is emulated instead of real.

[yuyichao]

This should be done with module aliases instead of syntax rewrite. The user should write using Compat.Test etc.

[yuyichao]

Also importall is being deprecated

[TotalVerb]

ok, I'll avoid rewriting importall then

[TotalVerb]

Module aliasing would force loading all the standard library modules as soon as Compatible loads, however.

[yuyichao]

They are anyway.

[TotalVerb]

Even so, module aliasing here introduces parts of Compat that will eventually need to be deprecated and migrated again, whereas the syntax rewriting can be removed later without deprecation.

[TotalVerb]

Alternatively, we could consider registering stub modules Test, SharedArrays, etc. that are restricted on METADATA for versions 0.6 and below? Though these version-conditional dependencies would not play nice with the existing REQUIRE format.

[yuyichao]

that will eventually need to be deprecated and migrated again

Yes? What's the problem? It doesn't change the fact that syntax rewrite is always the least preferred way to handle version compatibility, especially for a non-syntax change.

registering stub modules Test, SharedArrays, etc

That can also work.

[TotalVerb]

I agree in principle, but it just seems more elegant to be able to do @compat using Test instead of using Compat.Test. Would someone else like to comment?

[TotalVerb]

Replaced by #404.