Move foreign import of encodeDouble# to Common/ tree.
New[12]: Implement encodeDoubleInteger and decodeDoubleInteger.

Add add these to the tests.
New3 : Only shortcut ops on 'SmallPos 0##' when other operand is big.
New*/ : Better parameter names for mkPair function.
New3/.../Natural.hs : Replace timesNatural with timesNaturalNewest.
Remove Simple and GMP versions for this perfbug branch.
Pare down code and tests for this perfbug branch.