First non-trivial change for a while... because this involves reading from the OS, we should cache as much as we can in memory. So, group membership checking is done immediately after reading a group name, as the group structure has already been populated.

This is behaviour cribbed from ls. Even though I've never personally used it, I'm sure someone has a use for it.

- s/syntax/phase
- no more boxed array syntax
- getopts fail implements Show
- option#expect now requires &str

This finally means that the list of columns is no longer fixed, which means it has to be generated somewhere. And guess where it got moved to? That's right, the options object! (see previous commits)

I'd like to have as few public methods on it as possible.

They get computed often enough for this to be worthwhile, and I have plans for using the extension even if it's not what's being sorted by.

And move options struct into its own module