Change time type to units:millisecond_t<int64_t>
Since we previously used whole ms as int for mantle_api::Time the idea came up to also use an int type instead of double as underlying type for the unit.
Advantanges:
- closer to previous implementation.
Disadvantages:
- One should not convert directly from units::time::second_t to the new type because in the cast the ms are lost (only whole seconds are converted)
- calculations with the other units types are not possible anymore (e.g. my_time + 10_ms) because of the different underlying type