Garbage Collectors

[IchorDev]

So, D’s default garbage collector is the one named “conservative” 
in DRuntime…
I see there’s also “manual” which doesn’t actually function as a 
GC, which is interesting.
Nothing says what ProtoGC is… so I guess it’s useless.
Has anyone ever published any custom GCs? A search through the 
dub package registry yielded nothing. How hard would it be to 
port (for instance) a Java GC to D’s interface?