SPECIALISE INLINE pragma

[Mikolaj Konarski]

> On the same topic, I also wrote a blog post simply explaining the
> essential things to know
> about the inliner and specialiser as I don't think they are generally
> appreciated. Comments welcome!
>
> http://mpickering.github.io/posts/2017-03-20-inlining-and-specialisation.html

LGTM. I'd propose to link to this from GHC manual.

I didn't know the bit about INLINE being ignored
on a loop-breaker with no warning and no way of changing
the loop-breaker. That probably explains puzzling
and counter-intuitive results of some alternative layouts
of INLINEs in the computation-intensive parts of my code,
at least since the time I provide unfoldings for all functions
and so discounts don't help GHC in picking the intended loop-breaker.
And I don't think this ignoring of the programmer's intent
wrt INLINE is documented in the usual places.