Stop using higher-order macros to declare arenas

[Zalathar]

The arenas used by rustc_hir and rustc_middle are declared via macros. But instead of invoking rustc_arena::declare_arena! directly, those crates declare a higher-order macro containing a list of types, and then invoke that macro with declare_arena! as an argument.

From what I can tell, the only reason for this arrangement is so that the list of types can also contain [decode] modifiers that produce a corresponding implementation of RefDecodable that decodes into the arena. But the number of types involved is small enough that it's easy to list them separately, and the advantage of doing so is that we can remove a layer of macro indirection, making it easier to navigate to the real code.

There should be no change to compiler behaviour.

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[Zalathar]

This will have textual conflicts with #159893.