Cache the number of data cons in DataTyCon and SumTyCon
This is a follow-up after faf60e85 - Make tagForCon non-linear.
On the mailing list @simonpj suggested to solve the
linear behavior by caching the sizes.
Test Plan: ./validate
Reviewers: simonpj, simonmar, bgamari, austin
Reviewed By: simonpj
Subscribers: carter, goldfire, rwbarton, thomie, simonpj
Differential Revision: https://phabricator.haskell.org/D4131