Explicitly Freeing Memory

[Maxime Chevalier-Boisvert via Digitalmars-d-learn]

> Unfortunately I don't have any good suggestions... I have been 
> avoiding
> depending on dtors in D because of the aforementioned issues 
> (and more),
> so I haven't had much experience in debugging dtor-related 
> problems in
> D.

I decided to just free everything explicitly:

https://github.com/maximecb/Higgs/blob/03931178deb5794bf27b1020542d102a08286c07/source/runtime/vm.d#L784

It seems to address my memory leak issue for the moment: `make 
test` uses about 1/3 the memory and runs more than twice as fast.