[MLton] style guide

[Matthew Fluet]

> Since there is renewed interest in the style guide, as a way of
> getting it out there, what do people think of making it a section of
> the User Guide right before the MLton extensions section?  At least
> some of it certainly applies to the conventions we use in the MLton
> structure.

I don't think it really belongs in the User Guide.  I imagine that the
majority of users aren't going to be hacking the compiler sources, so it
doesn't really apply.  We could probably migrate the bit that discusses
sig/struct conventions into an intro the the MLton structure extension.