This exercise has always [1] required the user to return seq[string],
but the scenario is better modelled with an enum.

This commit is similar to previous overhauls for the allergies [2] and
space-age [3] exercises.

Closes: exercism#536

[1] 8015636, 2019-02-24, "Adds secret-handshake exercise"
[2] a8ab9c8, 2023-01-11, "exercises(allergies): overhaul to use enum"
[3] b770d96, 2023-01-11, "exercises(space-age): overhaul to use enum"

Similar to previous discussions [1] I probably have a small preference
for using shifts here, but it's fine either way.

Note that bitops2 calls this `getBit` [2].

[1] exercism#434 (comment)
[2] https://github.com/status-im/nim-stew/blob/3159137d9a31/stew/bitops2.nim#L420-L424

Suggested-by: ynfle

This better supports adding new enum values, as long as those values
aren't "special" like Reverse.

For symmetry with the branch above.