[MLton] Vector.fromList[] performance

[Stephen Weeks]

> > One possible advantage of putting Vector_fromList into MLton and
> > teaching MLton's optimizer about vectors would be that the SSA
> > optimizer could take advantage of their immutability.  Recovering that
> > from the array-cast-to-vector might be tricky.
> 
> You mean eliminating vector subscript operations in favor of the 
> vector elements known at creation?

Yeah.  Just another complexity that goes away if we deal with
Vector.fromList more directly.