We use the same trick in the C++ version of this function.
I measured the impact using `lean --new-frontend core.lean` and checking
the number of instructions executed reported by Valgrind.
Before: 4,891,642,264
After: 4,847,313,330
@kha, `eqn_compiler.lemmas` is false by default.
I will keep them disabled until I remove the inductive compiler.
I'm building the new inductive datatype module (to replace the inductive
compiler), and the lemmas will fail to be proved in the next commits
until the transition is complete.
We need this procedure otherwise it takes forever to prove equation lemmas
for definitions such as:
```
def macros : name → option macro
| `lambda := some lambda_macro
| `intro_x := some intro_x_macro
| _ := none
```
We never experienced this problem in Lean3 because we used `name`
literals only occurred in patterns of *meta* definitions. So, no
equation lemma was generated.
@kha `def macros` was taking more than 1 second to elaborate on my
machine. It is now instantaneous.
