New4/GHC/Integer/Prim.hs : Add function log2Word.
Check/Helpers.hs : Add function log2WordSlow to compare against fast log2Word.
New[123]/GHC/Integer/*.hs : Copy Prim.hs from New4 part of tree.

All New* versions of the Integer implementation now use the same Prim
module fore easier maintenance and performance comparison.
Check/NewX.hs.tpl : Reduce required processing.