Version 1.4.8

Changes in this version:

• Add support for % for modulo as an alternative to mod. Since % is also used for percentages, the way the operator is interpreted depends on context: 5 % 2 or (348*34)%(293-1) is parsed as modulo, whereas 5% or 5% + 3 continues to be treated as a percentage.
• Add constants electron_mass, proton_mass and neutron_mass