Shared toom evaluation functions

[Niels Möller]

I've worked some more with shared evaluation functions, for evaluation
in the points ±1 and ±2. To be able to share functions between toom43
and toom44, I also changed points for toom_interpolate_7pts to use -2
instead of -1/2.

Net reduction in source code is 150 non-comment lines, from 2438 to
2288 for all toom*.c. Net reduction in object size, on x86_32, is 2166
bytes, from 35158 to 32992 for toom*.o.

Patch is rather large, so I put it at
http://www.lysator.liu.se/~nisse/archive/toom-eval.patch

If you agree this is the way to go, I'd like to get this in before
further work on allocation cleanup.

I'm not sure how to benchmark to exclude performance regressions. I've
tried ./speed -s 500 mpn_mul_n, with no noticable difference.

/Niels