(PUP-3654) Deprecate Puppet.newtype

The `newtype` class method of the Puppet module includes a comment from
2011 stating that the method should be deprecated, but was under heavy
use at the time.

This commit adds a deprecation warning to this method to indicate that
it will be removed in a future release. Types using the old method have
also been updated to use `Puppet::Type.newtype`.