(maint) Change name from from_pson to from_data_hash

This is because these methods are also used for deserialization from
other formats than PSON. The method for serializing a object is called
to_data_hash, so makes sense to call this from_data_hash.
(maint) Deprecate from_pson methods

They have all been replaced with from_data_hash methods instead.
(maint) Unify to_pson in FormatSupport module, just like with to_msgpack

This removes all unnecessary to_pson definitions and creates a common
one in the Puppet::Network::FormatSupport module instead just like with
to_msgpack.

There are still some to_pson definitions left but that is because they
also add a document type structure. That cannot be removed until the
next major version as it would change the on wire format.