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.
