Mark Wooding pointed out that my comment in make1305.py was completely
wrong, and that the stated strategy for reducing a value mod 2^130-5
would not in fact completely reduce all inputs in the range - for the
most obvious reason, namely that the numbers between 2^130-5 and 2^130
would never have anything subtracted at all.

Implemented a replacement strategy which my tests suggest will do the
right thing for all numbers in the expected range that are anywhere
near an integer multiple of the modulus.