Create named struct `UnicodeVersion` to use instead of tuple type for
`UNICODE_VERSION` value. This allows user to access the fields with
meaningful field names: `major`, `minor`, and `micro`.
Per request, an empty private field is added to the struct, so it can be
extended in the future without API breakage.
Use `u32` for version components, as `u64` is just an overkill, and
`u32` is the default type for integers and the default type used for
regular internal numbers.
There's no expectation for Unicode Versions to even reach one thousand
in the next hundered years. This is different from *package versions*,
which may become something auto-generated and exceed human-friendly
range of integer values.
