[MLton] ref flattening

[Stephen Weeks]

I am pleased to report progress on the ref flattening optimization.  I
have checked in code that now passes all the regressions and the
benchmarks.   It does not yet self compile.  Here are the run time
ratios for the benchmarks run "{-drop-pass refFlatten,}".  I've only
included ratios that differ by more that 0.03 from 1.

MLton0 -- mlton -drop-pass refFlatten
MLton1 -- mlton 

run time ratio
benchmark         MLton1
barnes-hut          0.84
boyer               0.78
fib                 0.94
knuth-bendix        0.91
lexgen              1.28
logic               0.94
matrix-multiply     1.05
mlyacc              1.10
mpuz                0.92
nucleic             1.30
output1             0.80
ratio-regions       0.95
tsp                 0.80