Generics api (#8)

* README.md: update doc badge/link to pkg.go.dev

* s/CreateDatabase/NewDB

* Cosmetics: rename variables, functions

* Simplify object callback func, remove queryState argument

* Use type parameter for client object

* Unexport AddToBin

* s/ClientProxy/Object

* s/UpdateForNewLocation/Update

* Rename functions

* ci: update to go1.18

* Fix typos s/radisu/radius

* Add DB.Attach, remove NewObject

* DB.Detach and DB.DetachAll

* Rewrite FindNearestInRadius as a closure

* s/Object/Proxy and improve module doc

* ci: s/travis/github-ci