Make as_fillable_container() and as_fill_item() not mutate their inputs

[wch]

This is a follow-up to #861 (comment). Also closes #577.

This PR makes it so that as_fillable_container() and as_fill_item() do not modify their inputs.

Fixes #865

[wch]

The purpose of this new context manager is to make it so the code inside the with private_seed_n(): statement is clearer -- it will have the same form as it would if were called without the with private_seed_n():.

[wch]

@schloerke On a related note, I was looking at the functions tag_prepend_class(), tag_remove_class() tag_add_style() because they also mutate their input objects. Then I realized that those functions now exist as methods on the Tag class in htmltools. The methods mutate the object and return the object, but that feels more OK than outside functions that mutate the input and return it.

Is there any reason not to switch to those methods from htmltools? That would remove the need for those functions, and also make the code feel less weird.

[gadenbuie]

@wch I was looking into switching to the .add_class() method for appending the fillability classes. I was coming at this from the perspective of not putting utility classes first in the markup. I started working on making that change earlier today, but if it makes sense to roll that into this PR, I think that'd be a good change.

Edit: I opened an issue to track/explain this comment: #865

[wch]

I've removed the tag_ functions in favor of the Tag methods, and also made the fill classes come at the end. This closes #865.

[schloerke]

• changelog entry