Time::at fails when when passing kwargs as second argument

[b-n]

I recently discovered this was breaking in airb:

>>> Time.at(1, in: "Z")
Traceback (most recent call last):
	2: from (airb):1
	1: from (airb):1:in at
TypeError (no implicit conversion of Hash into Integer)

This should work:

[3.1.2] > Time.at(1, in: "Z")
=> 1970-01-01 00:00:01 UTC

I suspect this is somewhere in here: https://github.com/artichoke/artichoke/blob/trunk/artichoke-backend/src/extn/core/time/trampoline.rs#L74-L96

Also, it feels like this was already a work around for dealing with #2091. Preference would be to fix #2091, which should hopefully enable the refactor of this code to be a lot more sensible.