[MLton] MLton GC and 64-bit port

[Matthew Fluet]

> > Those are some intial thoughts, and may provide a starting point for
> > some discussion.
> 
> Just a short note, before going to bed: It's probably desirable (if
> not necessary) to ensure proper alignment of 64 bit objects (doubles,
> integers, pointers).  This might complicate things a bit more.

Not much.  For the Sparc, we needed to align 64-bit floating point values 
with 64-bit alignment, so the infrastructure is already there.  It may 
need to be revised to align yet more objects.