Mechanism for hand-written, but not wired-in, Ids

We got into terrible trouble trying to make unsafeCoerce# into a wired-in Id. We needed to make unsafeEqualityProof and UnsafeEquality wired in -- and the lattter is a GADT which is hugely painful to wire in.

New idea. Implement

patchMagicDefns :: [(Name, DsM (Id, CoreExpr))]
                -> [(Id,CoreExpr)]
                -> DsM [(Id,CoreExpr)]

and use it in the desugarer, here

        ; (msgs, mb_res) <- initDs hsc_env tcg_env $
                       do { ds_ev_binds <- dsEvBinds ev_binds
                          ; core_prs <- dsTopLHsBinds binds_cvr
--->  here --->           ; core_prs <- patchMagicDefns magicDefns core_prs
                          ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs

So now we could have

mmagicDefns :: [(Name, DsM (Id,CoreExpr))]
magicDefns
  [ ( unsafeCoerceName, do { unsafe_eq_proof <- dsLookupGlobal unsafeEqualityProofName
        ... )
  ]

The big thing is having access to dsLookupGlobal which means you don't need to wire in the trnansitive closure.