[MLton] fields at the front of GC_state

[Stephen Weeks]

> Why not just introduce an extra struct that groups the high-frequency 
> fields. This should be flattened out by the C compiler. You can keep the 
> fields within the new struct alphabetized. More importantly, you can 
> document that these set of frequently used fields should be treated 
> specially.

It feels like unnecessary verbosity to me.  All the C code that
accesses these will be continually reminded that these are
high-frequency fields, a fact which is usually irrelevant.  I guess we
could use macros to keep the verbosity down.