Drop Support for GHC < 8.6 in Preparation for PrimUnaligned

Before GHC 8.6, there are no primitives for unaligned access to byte arrays. These have now been available for a long time, and I have a typeclass and functions for this in the `primitive-unaligned` library at (Data.Primitive.ByteArray.Unaligned)[https://hackage.haskell.org/package/primitive-unaligned-0.1.1.2/docs/Data-Primitive-ByteArray-Unaligned.html]. The design of the typeclass needs to be changed soon since GHC is gaining primops for unaligned `Addr#` access, but I'd like to go ahead and start down the path of getting these consolidated. The first step toward that is removing support for GHC < 8.6.

Please keep discussing here about dropping support for GHC 8.0, 8.2, and 8.4. Any discussion about unaligned access to primitive arrays should happen at https://github.com/haskell/primitive/issues/409