Faster table compute in mpn_sec_powm

[paul zimmermann]

       Torbjörn,

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

this is a classical trick we use at several places in MPFR, and we mention
in Modern Computer Algebra (page 142).

Does someone has an earlier reference for that trick?

Paul