* expr: support arbitrary precision integers
Instead of i64s we now use BigInts for integer operations. This means
that no result or input can be out of range.
The representation of integer flags was changed from i64 to u8 to make
their intention clearer.
* expr: allow big numbers as arguments as well
Also adds some tests
* expr: use num-traits to check bigints for 0 and 1
* expr: remove obsolete refs
match ergonomics made these avoidable.
* formatting
Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>