This is an API breaking change.

Before this change, issues such as faker-ruby#1280 existed due to some
confusion about the behavior of Faker::Time.between. In essence,
between could produce values outside of the range of (from..to)
if the range is smaller than the provided period.

After this change, between now behaves more like Faker::Date.between
and as such the period argument has been removed. For the old
behavior, a user should switch to calling between_dates, which has
the same method signature as the old between method. The :between
period is no longer valid.

Documentation still needs to be updated, and additional tests need
to be written.

This is an API breaking change.

Before this change, Faker::Time re-used some behavior of Faker::Date
by first inheriting from Date, then using `super`. This relationship
was one of convenience, though, and created some odd behavior for
Time. For example, calling Faker::Time.birthday was possible, as
birthday is defined in Faker::Date, but it would unintuitively return
a Date object.

Now, Faker::Time explicitly calls out to Faker::Date methods
everywhere necessary, and the inheritance relationship has been
removed. This does remove Faker::Time::birthday, and
Faker::Time::between_except from the interface - but as these methods
both return Date objects and are not documented in Faker::Time docs,
they were never indicated for direct use anyway.

Adds a new test to prevent regression of faker-ruby#1280, and adds return type
checking for new method between_dates.

…o rework-time-between