Use the scale as the tick key function.

Previously we were using string coercion as the key function for axis ticks.
However, when the stringified value of the tick does not fully capture the
representation (such as a date with millisecond precision, whose string form
only has second precision; fixes #1529), string coercion is insufficient.

Fortunately, there is an equivalently-simple key function for tick identity: the
scale! If the scale does not return a unique position for the given tick, then
the tick would be overlapping, so it serves perfectly as the key function.