Type mismatch describer should merge Enum expectations

A function can be described with different dispatchers for different Enum values. An example of this is the recently introduced type function that uses three different dispatcher where each accepts one Enum choice. At present, this results in a fairly long message when passing something that isn't accepted. Example:

'type' parameter 'inference_method' expects a match for [#<Puppet::Pops::Types::PEnumType:0x000000032a33b8 @values=["detailed"]>, #<Puppet::Pops::Types::PEnumType:0x00000002e2b718 @values=["reduced"]>, #<Puppet::Pops::Types::PEnumType:0x00000002e16ef8 @values=["generalized"]>], got 'elaborated'  at line 1:8 on node test.example.com

There are two problems here. The first one is that the string representation that the describer uses for the Enum type is out of whack. The second is that it the message would be much clearer if it just mentioned one Enum but with several choices, e.g.

'type' parameter 'inference_method' expects a match for Enum[detailed,reduced,generalized], got 'elaborated' at line 1:8 on node test.example.com