Into<Number> for any number type

[Ballasi]

For now, Into<Number> only allows i64 and f64.

This has been chosen as this is the largest common number type that features the greatest range of numbers.

However, people may work with other types and are fine using smaller number ranges.

This forces the user to cast it to the 64 bits version of the number.

This issue is a bit connected to #5 as if the int/float type is removed altogether with another it may need to add Into case for each common number types.