Given a definition `foo`, they were previously called `foo._unfold`
until 4.7.0. We tried to rename them to `foo.def`, but it created too
many issues in the Mathlib repo. We decided to rename it again to
`foo.eq_def`. The new name is also consistent with the `eq_<idx>`
theorems generated for different "cases". That is, `foo.eq_def` is the
equality theorem for the whole definition, and `foo.eq_<idx>` is the
equality theorem for case `<idx>`.
cc @semorrison
Reason: avoid unnecessary case-analysis explosion
It is also unnecessary since we only refine the `below` and `F`
arguments over `match`-expressions.
This fixes the last case at issue #998
```
attribute [simp] BinaryHeap.heapifyDown
```
closes#998
