Faster algorithm for addition of coercions.
Faster algorithm for addition of coercions.

Instead of crawling a set of pairs, we maintain an indexing structure for
each component of the pair and use the corresponding subset when needed. The
underlying algorithm is still quadratic in the worst case but it should be
faster on average.

coq/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,coq/coq,SkySkimmer/coq,SkySkimmer/coq,ejgallego/coq,Zimmi48/coq,coq/coq,Zimmi48/coq,coq/coq,ejgallego/coq