Validator names

Minor thing, but personally I like to destructure my prop-types, and those names conflict with the validator names exported by this lib:

https://github.com/dhis2/ui-forms/blob/master/src/validators/index.js#L1-L20

So say you have this:

import { string } from @dhis2/prop-types
import { string } from @dhis2/ui-forms

I'm currently aliasing the ui-forms imports to isString, isBoolean, etc, on import. Since we already have hasValue there, I wouldn't find it weird to use that naming scheme for all validators. We could add them as aliases, and drop the original ones in the next major version.

As I said, very minor, but just a thought.