Rename Library.allClasses; tidy other collections

[srawlins]

A few tidyings to prepare for a bigger Library change I'm working on:

• Move all of the library collections (declared in TopLevelContainer) to one region in the file.
• Extract out common code, since they're all defined almost the same way.
• Rename allClasses to classesAndExceptions. Every time I came across Library.allClasses and, for example, Library.mixins, I've thought, "why all classes?" It just turns out that it is classes-that-don't-subclass-Error-orException, and classes-that-do. Having the shared combination is still useful; I just renamed it.
• Move Library.typeSystem over to ElementType, since it is only ever used over there.

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