epix_slide allows time_value and version to be different types & do unintended things

[brookslogan]

E.g., tsibble::yearweek and integer (yearweeks are probably treated like number of weeks since some epoch; assuming this is the case since tsibble::yearweek(int) appears to work this way rather than e.g. parsing approach of epidatr). We do want to allow mixed types at some point; e.g., time_values representing weeks and versions representing datetimes. Datetime versions are likely more complicated, as, e.g., they probably won't have a regular period.