Minimise provided dictionaries in pattern synonyms
Trac #14394 showed that it's possible to get redundant constraints in the inferred provided constraints of a pattern synonym. This patch removes the redundancy with mkMinimalBySCs. To do this I had to generalise the type of mkMinimalBySCs slightly. And, to reduce confusing reversal, I made it stable: it now returns its result in the same order as its input. That led to a raft of error message wibbles, mostly for the better.