Add @check and @should_not_warn.

[jamii]

The intended usage is to tag inner loops with @should_not_warn and
then run existing tests inside @check to catch any regressions in type-stability.

[jamii]

@MikeInnes sorry, I requested you by accident and there is no way to unrequest :D

[jamii]

Relevant discussion - JuliaLang/julia#10980

[pfitzseb]

This generally looks good, but I don't like decorating the function definition with @should_not_warn much -- what do you thing of changing @check such that it also accepts a function (or vector of functions) that are not allowed to warn? We could then also have @check funcall(args) default to failing on all thrown warnings (basically #21).
Not having the decoration macro would also allow Traceur.jl to be a test-only dependency.

[jamii]

I'm open to that. I expect we'll end up adding @should_not_warn to our codebase anyway, because:

• all calls to @check will be using the same set of functions
• annotating the function is a useful reminder when editing it

But it does make sense for it to avoid dragging Traceur in as a dep.

[pfitzseb]

Alright, then I think this should be good to go and we can add the other functionality in a separate PR. Will merge this tomorrow barring any objections.