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When examining compile times for code with large ADTs (particularly those with many record constructors), I found that the specialiser contributed disproportionately to the compiler runtime. Some profiling suggested that the a great deal of time was being spent in `pair_fvs` being called from `consDictBind`. @simonpj pointed out that `flattenDictBinds` as called by `specBind` was unnecessarily discarding cached free variable information, which then needed to be recomputed by `pair_fvs`. Here I refactor the specializer to retain the free variable cache whenever possible. **Open Qustions** * I used `fst` in a couple of places to extract the bindings from a `DictBind`. Perhaps this is a sign that `DictBind` has outgrown its type synonym status? Test Plan: validate Reviewers: austin, simonpj Reviewed By: simonpj Subscribers: thomie, bgamari, simonpj Differential Revision: https://phabricator.haskell.org/D1012 GHC Trac Issues: #7450
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