This PR removes the auto-generated `binductionOn` and `ibelow`
implementations for inductive types in favor of the improved `brecOn`
implementation from #7639.
code to create nested `PProd`s, and project out, and related functions
were scattered in variuos places. This unifies them in
`Lean.Meta.PProdN`.
It also consistently avoids the terminal `True` or `PUnit`, for slightly
easier to read constructions.
We now get `.below` and `.brecOn` definitions for nested inductives.
No surprises in the implementation: the kernel already gives us suitable
`.rec_1` etc. recursors, and our construction follows the structure of
this recursor.
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Co-authored-by: Tobias Grosser <tobias@grosser.es>
