Revisit Implementation of Offset Numerator and Denominator in Units Conversion

[younies]

Description

In the experimental/unitsconversion/src/provider.rs file, we have the following fields in the ConversionInfo struct:

/// Represents the numerator of the offset.
#[cfg_attr(feature = "serde", serde(borrow))]
pub offset_num: ZeroVec<'data, u8>,

/// Represents the denominator of the offset.
#[cfg_attr(feature = "serde", serde(borrow))]
pub offset_den: ZeroVec<'data, u8>,

from @sffc :
Since these are usually unused, I wonder whether we can represent them more compactly. Currently they get an entry in the multi-field ULE that #[make_varule] generates.