New handling of overlapping inst in Safe Haskell

We do much better now due to the newish per-instance flags. Rather than
mark any module that uses `-XOverlappingInstances`,
`-XIncoherentInstances` or the new `OVERLAP*` pragmas as unsafe, we
regard them all as safe and defer the check until an overlap occurs.

An type-class method call that involves overlapping instances is
considered _unsafe_ when:

1) The most specific instance, Ix, is from a module marked `-XSafe`
2) Ix is an orphan instance or a MPTC
3) At least one instance that Ix overlaps, Iy, is:
   a) from a different module than Ix
   AND
   b) Iy is not marked `OVERLAPPABLE`

This check is only enforced in modules compiled with `-XSafe` or
`-XTrustworthy`.

This fixes Safe Haskell to work with the latest overlapping instance
pragmas, and also brings consistent behavior. Previously, Safe Inferred
modules behaved differently than `-XSafe` modules.

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