Allocgate

[leecannon]

It was noticed in #10037 that LLVM is not able to properly optimize interfaces that use @fieldParentPtr as per @SpexGuy's comment.

The Random interface was changed in #10045 to instead use a type erased pointer in place of @fieldParentPointer resulting in significant performance improvement.

This issue is to track applying the same change to Allocator, however due to this interface being much more widely used it is planned to be merged just before the 0.9.0 release.

Soon I will open a draft PR to begin working on this.

[iacore]

I think we should add a unified way for implementing polymorphism while we are at it. #10184