RFC: Implement test for gc-managed allocation

In optimizing code, usually one of the most important steps is to stamp out any unnecessary garbage-collection. One can test for this with the profiler, but a more direct approach seems desirable.

Demonstration:
```
julia> x = randn(3,5,8);

julia> size(x)
(3,5,8)

julia> size(x,2)
5

julia> @gcallocated sz = size(x,2)
false

julia> @gcallocated sz = size(x)
true
```

In the source, I'm not certain I got every place where this is relevant, but I hope I did.