Unary minus and bitwise ops (shift, xor, ...) are not supported by GNU gettext.
Furthermore once can easily run into undefined behavior when using those
and there doesn't seem to be a valid use in determining the plural form
using those.
Unary minus is kept as it is useful for testing at least.
The parser and evaluator of plural expressions in PO/MO files isn't
really tested thorouhly.
Add tests for expressions from some languages as well as some basic
tests for all possible operations comparing those against the result of
calculating the value in C++.
