[loaders] improve fixed-width saver

[daviewales]

Don't add so many spaces between columns when saving.
Use the max width of the data, rather than the max width of the window when saving.
Don't include the separating space as data when loading. (Prevents extra space being added each round trip, as the saver also adds a single space between columns.)

Fixes #2255.

[daviewales]

This could also be:

            yield colstart, i-1 #2255

prev+1 collapses multiple spaces between columns.
i-1 assumes only a single space is the column separator, and keeps the remaining spaces as data.

For example, given the following fixed width data with three spaces between the columns:

column_1    column_2
one         two

prev+1 would be saved as the following CSV. (Separating spaces are removed, but padding spaces are retained):

column_1,column_2
one     ,two

i-1 would be saved as the following CSV. (One separating space is removed. All others are retained.):

column_1  ,column_2
one       ,two

The current version of the code with prev+1 is my personal preference, but open to guidance if the i-1 version better matches the intent of 'fixed width'.

[daviewales]

Note, it must be i-1 rather than i to avoid adding an extra space between the columns each round trip.

[saulpw]

Okay, I prefer prev+1 too. Thanks for the explanation.

[daviewales]

I think this is ready for initial review. Let me know if you're happy with the suggested changes, and I'll update the tests accordingly. (They are currently broken due to these changes.)

[daviewales]

Note: Something which should be considered is a file like the following:

colours shades               counts 
red     light                3      
red     dark                 4      
green   very light           5      
green   very very very light 5      
blue    dark                 8      

VisiData reads this as four columns rather than three, because it splits columns based on unbroken vertical space, rather than header position.

This behaviour remains unchanged by this PR.

[saulpw]

Thanks for your attention on this, @daviewales! If you wouldn't mind updating the tests too, we'd much appreciate it.

[anjakefala]

Hey @daviewales!

I went ahead and made the requested changes, and updated the tests. =) Thanks for taking care of this.

[daviewales]

Thanks @anjakefala, much appreciated!