Experimental switch to iterator based api

[CosmicHorrorDev]

Overall note: I'm planning on leaving the docs broken/outdated until all the large changes are done. No need to fix them just to have them break again with the next PR

This is an experimental look at a redesign around an iterator based API. The main aspects that changed are:

• LibraryFolders is now represented by a Vec<Library> where each Library has its path and apps that it contains
• Library::apps() returns an AppIter that iterates over each manifests and attempts to parse it
• There is no longer an SteamDir::apps (I think I still want to provide this, just thinking on how to do it)
• Switched a lot of things to private with public accessors

I added an overview example to show how the changes look. Here's the (modified) output of that for my laptop

Steam Dir - "$HOME/.steam/steam"
    Library - "$HOME/.data/Steam"
        App 677120 - Heroes of Hammerwatch
        App 1493710 - Proton Experimental
        App 620 - Portal 2
        App 477160 - Human: Fall Flat
        App 341800 - Keep Talking and Nobody Explodes
        App 1092790 - Inscryption
        App 440 - Team Fortress 2
        App 340 - Half-Life 2: Lost Coast
        App 400 - Portal
        App 70 - Half-Life
        App 1714040 - Super Auto Pets
        App 228980 - Steamworks Common Redistributables
        App 1274570 - DEVOUR
        App 457140 - Oxygen Not Included
        App 48000 - LIMBO
        App 398850 - Epistory - Typing Chronicles
        App 698640 - Deep Sky Derelicts
        App 628670 - Hellpoint
        App 246110 - MASSIVE CHALICE
        App 220 - Half-Life 2
        App 1391110 - Steam Linux Runtime - Soldier
        App 322330 - Don't Starve Together
        App 247080 - Crypt of the NecroDancer
        App 208750 - Apotheon
    Library - "/path/to/other/lib/temp steam lib"
        App 1794680 - Vampire Survivors
        App 4000 - Garry's Mod

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That's all for now. Just figured I would put this up for some early feedback