The `quot` type is now implemented in the kernel.
We will do the same thing for inductives.
We will not support normalizer extensions anymore in Lean4.
It doesn't make sense since we settled with 2 extensions: quotients and
inductives. Moreover, any new extension would require substantial
changes (e.g., code generator).
The normalizer_extension feature was useful when we were experimenting
with different kernel flavors.
