Faster table compute in mpn_sec_powm

[Torbjörn Granlund]

  B^(2j)   = (B^j)^2      (i.e., even values of i)
  B^(2j+1) = (B^(2j))*B   (i.e., odd values of i)

I pushed a set of changes implementing this:

https://gmplib.org/repo/gmp/raw-rev/186ea964d61f
https://gmplib.org/repo/gmp/raw-rev/a6967ea18b2f
https://gmplib.org/repo/gmp/raw-rev/289ab13bc3fe
https://gmplib.org/repo/gmp/raw-rev/cd4716561151

(The first two are cleanups, the latter two implement the actual
change.)

I haven't benchmarked this change, but it is clearly never a slowdown.

-- 
Torbjörn
Please encrypt, key id 0xC8601622