Upgrade to Go 1.20 and use stdlib for everything (#15)

* Upgrade to Go 1.20 and use stdlib for everything

With Go 1.20, it's feasible to remove all third party dependencies from the library (which will cascade into complete and your other packages).

* Drop named returns

* Add a test for merr returns

* Incorporate review feedback

Add iterate

v1.1.0

Update readme according to Go doc

v1.0.0

fix modifier doc