Specialise: Avoid unnecessary recomputation of free variable information

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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