New[34]/GHC/Internal/WordArray.hs : Drop use of pinned memory.

Pinned memory has negative consequences for garbage collection. It
also allowed the use of a new track/hack when implementin Karatsuba
multiplication but the negative consequences are not worth it.

Funnily enough, dropping the use of pinned memory had a significant
performance improvement.
Check/Helpers.hs : Fix log2WordSlow implementation.
Move New1/GHC/Integer/Prim.hs -> Common/GHC/Integer/Prim.hs.

Since the Prim module is almost identical across all 4 New*
implementations, rationalize into a single commont version.
Rename New3/GHC/Integer/StrictPrim.hs -> Common/GHC/Integer/StrictPrim.hs

And rationalize to a single common version of this file.