Optional monitors suggestion

[bearophile via Digitalmars-d]

Yuriy:

> Meanwhile, i've updated the PRs according to latest notes. 
> @monitor attr, forbid __monitor declaration, so everyone can 
> have some fun with it. =)

You are good. Before this is ready for Walter's judgement I think 
this needs some simple performance/memory benchmarks, to compare 
the situation before and after this change.

Bye,
bearophile