std.allocator ready for some abuse

[bearophile]

Andrei Alexandrescu:

> and morph and combine in infinite ways.

Yes, those combination capabilities look very nice :-)

I presume this module is not in "core" because it's meant to be 
used from normal D code.

- Could some new/extra D language/compiler support/features help 
this module usage safety, efficiency, expressibility or usage 
syntax (beside Issue 11331)?
- Can you create an hierarchical allocator with those?
- Are some operating system memory functions (below C malloc) 
used/supported/addable/useful for those allocators? (Including 
awareness of virtual memory management from the OS).

Bye,
bearophile