Document which casts xcast() supports for hms, Date and POSIXct

[joethorley]

Closes #38.

xcast() passes the conversion to vctrs::vec_cast(). The hms package provides casts from numeric and character values to hms, but vctrs deliberately does not provide casts from numeric or character values to Date or POSIXct as they require an origin, format or time zone.

• Explains this in the xcast() details and says that Date and POSIXct values must be created with the required class (e.g. as.Date(), as.POSIXct()), and that POSIXct values are converted to the time zone of the column.
• Adds examples casting to hms from character and supplying Date and POSIXct values.
• Adds a test that Date and POSIXct values are accepted and that POSIXct keeps the same instant in the column's time zone.

No behaviour change.

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