Move GreName, split up GHC.Types.Name.Reader

Following !4467 (closed) the GHC.Types.Name.Reader module should be split up. See !4467 (comment 316521), wherein quoth @simonpj:

I would group together

• data GlobalRdrElt and the types it depends on
• data GREName into one module, maybe GRE.hs or GlobalRdrElt.hs. That would leave a much slimmed down Reader.hs, some of which might disappear into Avail.hs. I'm not sure what that leaves. But GHC.Types.Name.Reader is a bad module name anyway. Maybe GHC.Types.RdrName?

Also !4743 (comment 323136):

PS: Maybe we should s/FieldGreName/FieldGre globally? Ditto NormalGreName.