[phobos] Gcx: Would we ever want more than one?

[David Simcha]

I'm looking to get rid of the global malloc lock for small memory 
allocations.  A major barrier to pulling this off within the current GC 
design is the fact that Gcx is a struct, which suggests the possibility 
of having more than one instance and makes it more difficult to create 
thread-local objects.  Is there any reason why we would ever want more 
than one garbage collector instance?  If not, why is Gcx a struct 
instead of just a bunch of __gshared variables?