Update to use alex >=v3.0.3

alex 3.0.3 makes 2 changes that affect yi's lexers:

- Changed format of `AlexAcc` datatype
- Conditionally includes the `*Pred` constructors for `AlexAcc`

This pul request updates yi to handle both those changes, and allows it to compile with alex 3.0.3 thru 3.0.5. It inadvertently dupilcates some of the work in pull request 435.