Scales with date / datetime input are broken

[anthonynorth]

Continuous scales should support dates and datetimes. Discrete scales should support dates. Underlying scales::train_continuous() is coercing POSIXt as numeric, and Date as integer.

[anthonynorth]

Edit: There are more blockers than just scales::trans_continuous(). POSIXt and Date vectors are serialised as text, so these will need to be fixed up in the client, possibly with just vec.map((x) => new Date(x))