Make FixedInt constexpr, fix ODR of fix<N>

Antonio Sánchezrequested to merge
cantonios/eigen:odr into master

Related to #2392 (closed), the use of a static variable in a header file leads to potential ODR violations. Removing static should resolve this (without inline) since it is a variable template. Also made this constexpr, which required making the FixedInt class constexpr-compatible.

Merge request reports