Desugar multiple polymorphic bindings more intelligently
Occasionally people write very large recursive groups of definitions. In general we desugar these to a single definition that binds tuple, plus lots of tuple selectors. But that code has quadratic size, which can be bad. This patch adds a new case to the desugaring of bindings, for the situation where there are lots of polymorphic variables, but no dictionaries. (Dictionaries force us into the general case.) See Note [Abstracting over tyvars only]. The extra behaviour can be disabled with the (static) flag -fno-ds-multi-tyvar in case we want to experiment with switching it on or off. There is essentially-zero effect on the nofib suite though. I was provoked into doing this by Trac #1136. In fact I'm not sure it's the real cause of the problem there, but it's a good idea anyway.