Iterator Rewrite: Part 1

[joeyballentine]

Part 1 of the Iterator Rewrite ™️. While not everything we want to be able to accomplish out of iterators, it works so much better than the current system.

Improvements:

• No more giant sub-editor behemoth nodes getting in your way
• Iterators and collectors can now be mixed and matched, for example to convert an image sequence to video
• Visual indicator on handles when something is iterable or collects an iterable sequence

Drawbacks:

• While you can mix and match iterators with collectors, you can only use one iterator per "lineage"
• no explicit mapping operations, or fancy things like take/every or filtering
• No type system information for iterators
• Still doesn't allow nodes to expand or contract the amount of items in an iterable, so frame interpolation is still not viable

Open Questions:

• Are the names of the nodes good?

Things that should be extensively tested:

• Regression of functionality. These should be identical to existing iterators
• Migration of old iterators to new
• The video iterator is a bit different, so we need to be sure I didn't break anything

Future work:

• There's a lot of old iterator code that I left in. I kinda don't wanna bog this PR down with removing all that, so I plan on doing that in a separate PR
• Expanding this system to fix the drawbacks

Issues that get closed as a result of this:

• Closes While you change of Chain using a Model File Iterator node, size of previous node is kept in memory #2120
• Closes 'Save Image' node can overlap Image Directory Iterator #1731
• Closes Pass a directory list to the image file iterator #1682
• Closes Iterator resizing is not recorded in history #1298
• Closes Unable to paste a copied/cut node(s) from the *iterator space outside of it #1355
• Closes Sprite sheet iterator and generator #564

[RunDevelopment]

Found some bugs.

1. You cannot connect to an already-connected input that is downstream of an iterator. Example:
   

2. Non-iterator outputs can be connected to iterator inputs.
   
   

[joeyballentine]

Non-iterator outputs can be connected to iterator inputs.

Without type information I dont think that can be avoided. I could just automatically convert those to arrays I guess?

[RunDevelopment]

I could just automatically convert those to arrays I guess?

Let's not make a bug a feature, okay? Since we should make iterators a type in the type system anyway, we can fix it in later PRs I guess.

On that note: we should make issues for the iterator things we want to change/fix.

[RunDevelopment]

LGTM; We still need to polish iterators a lot, but this is a solid foundation. Let's merge it!