Make recursive_add/accumulate more recursive

[danielwe]

This is to explore functionality for realizing JuliaMath/QuadGK.jl#120. The current draft cuts time and allocations in half for the MWE in that PR compared to the make_zero hack from the comments. Not sure if modifying the existing recursive_* functions like this is appropriate or whether it would be better to implement a separate deep_recursive_accumulate.

This probably breaks some existing uses of recursive_accumulate, like the Holomorphic derivative code, because recursive_accumulate now traverses most/all of the structure on its own and will double-accumulate when combined with the iteration over the seen IdDicts. Curious to see the total impact on the test suite.

This doesn't yet have any concept of seen and will thus double-accumulate if the structure has internal aliasing. That obviously needs to be fixed. Perhaps we can factor out and share the recursion code from make_zero.

A bit of a tangent, but perhaps a final version of this PR should include migrating ClosureVector to Enzyme from the QuadGK ext as suggested in JuliaMath/QuadGK.jl#110 (comment). Looks like that's the most relevant application of fully recursive accumulation at the moment.

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Let me also throw out another suggestion: what if we implement a recursive generalization of broadcasting with an arbitrary number of arguments, i.e., recursive_broadcast!(f, a, b, c, ...) as a recursive generalization of a .= f.(b, c, ...), free of intermediate allocations whenever possible (and similarly an out-of-place recursive_broadcast(f, a, b, c...) generalizing f.(a, b, c...) that only materializes/allocates once if possible). That would enable more optimized custom rules with Duplicated args, such as having the QuadGK rule call the in-place version quadgk!(f!, result, segs...). Not sure if it would be hard to correctly handle aliasing without being overly defensive, or if that could mostly be taken care of by proper reuse of the existing broadcasting functionality.