[ fix #758 ] desugar non-binding sequencing in do blocks to (>>)

[gallais]

Instead of desugaring do blocks using >>= only i.e. elaborating

do x <- p
   q
   r

to

p >>= \ x => q >>= \ _ => r

we make sure that non-binding sequencing uses (>>) and get:

p >>= \ x => q >> r

This allows us to have a special >> that, for instance, is compatible
with a linear bind by enforcing that its first argument returns (),
a value easily consumed.

[andrevidela]

This seems like quite a disruptive change, doesn't it break backward compatibility too? Not having to bind resulting values from in do block was a nice quality of life feature that we lose.

[gallais]

doesn't it break backward compatibility too?

Yes. On the other hand, it helps inference and you get a warning when
you're silently throwing away a non-unit result (you can use ignore
to throw it away explicitly).