Move predicates into `predicates` package

The predicates in refined are currently organized in objects (like `numeric`, `string`, etc.) in the `eu.timepit.refined` package. With the addition of predefined refined types in `eu.timepit.refined.types` the predicates became a little bit less important (since now if you want to use `NonEmptyString` for example, the only thing you need to import is `eu.timepit.refined.types.string.NonEmptyString`). It is also unfortunate that we have `eu.timepit.refined.string` and `eu.timepit.refined.types.string` while it is not obvious from the package name that the former only contains predicates.

So to clean up the package structure, I'd like to move all predicates to `eu.timepit.refined.predicates` so that predicates are on the same level as refined types. The `.refined.predicates` package is already there but currently only contains forwarders to the actual predicates in `refined` and an `all` object which allows to import all predicates (to support #184).

Care should be taken when doing this to minimize breakage of binary and source compatibility.