Complete work on new OVERLAPPABLE/OVERLAPPING pragmas (Trac #9242)
* Deprecate -XOverlappingInstances * Update test suite. Several tests even had entirely unnecessary uses of -XOverlappingInstances * Update user manual with a careful description of the instance resolution story * Fix an outright bug in the handling of duplidate instances in GHCi, which are meant to silently overwrite the earlier duplicate. The logic was right for family instances but was both more complicated, and plain wrong, for class instances. (If you are interested, the bug was that we were eliminating the duplicate from the InstEnv, but not from the [ClsInst] held in tcg_insts.) Test is ghci044a.