Previously, we declared command arguments with an example of
their value. After this commit, we declare command arguments
as a dict of metadata. This allows us to define the value
type, whether the argument is required, and provide default
values. This in turn allows us to have nice things, such as
less boilerplate code in individual commands for validating
input and assigning default values. It should also make command
behavior more consistent as a result.
Test output changed slightly because I realized that the "fields"
argument wasn't being consistently defined as a set. Oops!
Other test output changed because of slight differences in code
performing type validation.
Shouldn't there be a default XOR required == true? That is, could we get away with default=None and then deriving requiredness from default-availability?