Axe RecFlag on TyCons.

Summary:
This commit removes the information about whether or not
a TyCon is "recursive", as well as the code responsible
for calculating this information.

The original trigger for this change was complexity regarding
how we computed the RecFlag for hs-boot files.  The problem
is that in order to determine if a TyCon is recursive or
not, we need to determine if it was defined in an hs-boot
file (if so, we conservatively assume that it is recursive.)

It turns that doing this is quite tricky.  The "obvious"
strategy is to typecheck the hi-boot file (since we are
eventually going to need the typechecked types to check
if we properly implemented the hi-boot file) and just extract
the names of all defined TyCons from the ModDetails, but
this actually does not work well if Names from the hi-boot
file are being knot-tied via if_rec_types: the "extraction"
process will force thunks, which will force the typechecking
process earlier than we have actually defined the types
locally.

Rather than work around all this trickiness (it certainly
can be worked around, either by making interface loading
MORE lazy, or just reading of the set of defined TyCons
directly from the ModIface), we instead opted to excise
the source of the problem, the RecFlag.

For one, it is not clear if the RecFlag even makes sense,
in the presence of higher-orderness:

    data T f a = MkT (f a)

T doesn't look recursive, but if we instantiate f with T,
then it very well is!  It was all very shaky.

So we just don't bother anymore.  This has two user-visible
implications:

1. is_too_recursive now assumes that all TyCons are
recursive and will bail out in a way that is still mysterious
to me if there are too many TyCons.

2. checkRecTc, which is used when stripping newtypes to
get to representation, also assumes all TyCons are
recursive, and will stop running if we hit the limit.

The biggest risk for this patch is that we specialize less
than we used to; however, the codeGen tests still seem to
be passing.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Reviewers: simonpj, austin, bgamari

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D2360

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