disallow string to bool casts

we always truncate, like in C

polars does not write them properly to the database

we do not guarantte equivalence here (e.g. 0.0 vs 0), but we guarantee that a
sequence of casts Float64 -> String -> Float64 is the identity (up to some
epsilon).

so that e.g. mssql can return DATETIME2 instead of DATETIME

we currently only guarantee this to work for datetimes of the form
YYYY-MM-DD HH:MM:SS.MILLIS (yes, exactly six digits after the comma, which makes
sense since we guarantee all operations on datetimes on millisecond precision)

we allow __eq__ between a dtype instance and a dtype class. When comparing with
a class, const and vararg are ignored.