[Yhc] Yhc using mtl

[Twan van Laarhoven]

Neil Mitchell wrote:
 >> But IMO FlexibleInstances are a noncontroversial extension, and they
 >> are supported by most compilers.
 >
 > Alas but not Yhc - which will break our goal of being self-compiling
 > at some stage. How much work is it to move to not requiring this? If
 > that could be done, then this patch will definately be applied.
 >

Getting rid of the requirement for FlexibleInstances should not be to 
hard (nor should adding support for them to Yhc). But even if I work 
around that, this patch relies quite havilly on the mtl, which uses 
multi parameter type classes and functional dependencies. Those might be 
a bigger problem. Hoever the mtl was already used in other parts of the 
compiler...

Twan