extern keyword (cf conditional modules)

[StefanKarpinski]

Implement the extern keyword as discussed here: #6884.

[tkelman]

thought this would have to be an 0.5 target, or am i not remembering the conversation on this right?

[StefanKarpinski]

It would be great but someone's got to do the work and I think it's a pretty short list of people who can.

[tkelman]

What about reserving the syntax as a no-op? Or will a macro in Compat be enough to let package authors start trying to use it once it gets implemented without breaking on past Julia versions?

[tkelman]

x-ref relevant discussion on the mailing list https://groups.google.com/d/msg/julia-users/RDTZnVbiBig/NfBTtodUBAAJ, particularly in how this needs to tie into precompilation

[tkelman]

This will be important to sit down and hammer out an implementation plan after #265 and the type system revamp (#8974) are finished.