Note [Resolving never-exported Names]

For the high-level overview, see
Note [Handling never-exported TyThings under Backpack]

As described in 'typecheckIfacesForMerging', the splendid innovation
of signature merging is to rewrite all Names in each of the signatures
we are merging together to a pre-merged structure; this is the key
ingredient that lets us solve some problems when merging type
synonyms.

However, when a 'Name' refers to a NON-exported entity, as is the
case with the DFun of a ClsInst, or a CoAxiom of a type family,
this strategy causes problems: if we pick one and rewrite all
references to a shared 'Name', we will accidentally fail to check
if the DFun or CoAxioms are compatible, as they will never be
checked--only exported entities are checked for compatibility,
and a non-exported TyThing is checked WHEN we are checking the
ClsInst or type family for compatibility in checkBootDeclM.
By virtue of the fact that everything's been pointed to the merged
declaration, you'll never notice there's a difference even if there
is one.

Fortunately, there are only a few places in the interface declarations
where this can occur, so we replace those calls with 'tcIfaceImplicit',
which will consult a local TypeEnv that records any never-exported
TyThings which we should wire up with.

Note that we actually knot-tie this local TypeEnv (the 'fixM'), because a
type family can refer to a coercion axiom, all of which are done in one go
when we typecheck 'mi_decls'.  An alternate strategy would be to typecheck
coercions first before type families, but that seemed more fragile.