@Kha
I kept `TokenInfo` as is. That is, the `lbp` field is still `Option Nat`.
I changed my mind because we have the combinator `NonReservedSymbol`.
It feels weird to have a combinator that does not create a keyword,
but sets the `lbp`. Recall that it is used at `Lean/Parser/Tactic.lean`.
Other observations:
- We use symbols in auxiliary constructions (e.g., `mkAntiquot`). It
feels weird to set their `lbp` there.
- It felt weird to specify the `lbp` in places such as
```
def structCtor := parser! ident >> optional inferMod >> symbol " :: " 67
```
- We have a few parsing rules where the same symbol appears twice.
It is funny to set the `lbp` twice. Note that the approach we
discussed yesterday (retrieving the `lbp` from the `Environment`)
would not work here.
