metric-space

Copperbox revision 835.

I've added a new library to represent metric spaces - i.e some set (Haskell Type) with an operation distance (d :: A -> A -> Real), though I've generalized the result from Real to an associated type.

I think I'll spare Hackage, at least until I have some noteworthy derived functions. My last contribution to Hackage - groupoid - was somewhat tardy.