The `offset` field is problematic because it prevents us from having an
efficient way of moving back and forth between `String.Pos` and `String.Iterator`.
@kha I temporarily added `String.OldIterator` for making sure the
parser doesn't break. This is a temporary fix that will be eliminated
after we replace `parsec`.
@kha It will be awesome to automate the following idiom with a macro :)
```
def defIndent := 4
def getIndent (o : Options) : Nat := o.get `format.indent defIndent
@[init] def indentOption : IO Unit :=
registerOption `format.indent { defValue := defIndent, group := "format", descr := "indentation" }
```
Now, the nodes in a `rbmap` contain the key and value, and we avoid
one level of indirection. `rbmap`s are more common than `rbtree`.
We implement `rbtree A` as `rbmap A unit`.
