This PR removes uses of `Lean.RBMap` in Lean itself.
Furthermore some massaging of the import graph is done in order to avoid
having `Std.Data.TreeMap.AdditionalOperations` (which is quite
expensive) be the critical path for a large chunk of Lean. In particular
we can build `Lean.Meta.Simp` and `Lean.Meta.Grind` without it thanks to
these changes.
We did previously not conduct this change as `Std.TreeMap` was not
outperforming `Lean.RBMap` yet, however this has changed with the new
code generator.
TODO: convert fixed indices to parameters. Motivation: types such as
```lean
inductive Foo : List α → Type
| mk : Foo []
```
Users should not need to write
```lean
inductive Foo {α} : List α → Type
| mk : Foo []
```
