[MLton] Slightly simpler functional record update

[Stephen Weeks]

> A further minor advantage of the above style, which is not
> necessarily immediately apparent, is that this style will be shorter
> (in terms of the number of characters) once the record field names
> get longer than 7 (or 5 if using single character ids) characters on
> average. So, I'll generate this style by default in
> `esml-gen-fru-setter' (unless there is a technical argument in favor
> of the other style).

Fair enough.  There is no technical reason.

> > BTW, for easier typing, how about "M x esml-gen-fru-setter a b c"?
> 
> Sure, no problem. The idea of allowing a record pattern or a type is that
> you can copy-paste it to the minibuffer.

Ah.  I'm not sure which is preferable then.  I guess it's easy enough
to support both?