Rewrite mul/sqr for 32bit/64bit

- interleave calculation of the lower and upper partial product ranges, and reduction
- less registers needed, more opportunities for parallel ops
Add equalities relating input and output variables
Add overflow analysis to field_5x52_int128_impl.h
Add overflow analysis to field_10x26_impl.h
Merge pull request #103

f8cce95 Add overflow analysis to field_10x26_impl.h (Pieter Wuille)
a518598 Add overflow analysis to field_5x52_int128_impl.h (Pieter Wuille)
fa0d620 Add equalities relating input and output variables (Pieter Wuille)
5dd421b Rewrite mul/sqr for 32bit/64bit (Peter Dettman)