[MLton] share weirdness

[Henry Cejtin]

Unix  is  very  careful  to always zero out memory given to you by the OS (to
keep you from being able to see something you shouldn't).  Still,  I  figured
out  afterwards  that  things  like  rusage calls would leave different stuff
around.

All of this kind of argues that either hashCons() should only check the  true
bytes  or else the background should be zero'd, just for repeatability and to
maximize sharing.  In this case,  it  must  be  strings  whose  size  is  not
divisible  by  4  that  is  leaving some bytes unset.  Forcing trailing zeros
might not be too bad here.  Still, it isn't clear.