factor::numeric::DoubleInt: Clarify methods and associated types
factor::numeric::DoubleInt: Clarify methods and associated types

- `DoubleInt::Double` renamed to `DoubleWidth`
- `{as,from}_double()` renamed to `{as,from}_double_width()`.

This should hopefully clarify that this is not a “double precision”
floating-point type, but an integer type with a larger range (used
for storing intermediate results, typ. from a multiplication)

uutils/coreutils,uutils/coreutils,uutils/coreutils