[Swan-dev] new function: show_set_short()

[D. Hugh Redelmeier]

This is like enum_show_shortb() except:

- instead of using a pointer to an array of pointers to element names 
  this uses an enum_names

- instead of using the full name of each element, it uses the "short name"
  (the name after the prefix is removed)

- the components making up the function's name are permuted.  This is to 
  make it clearer that "set" is a noun, not a verb.

This was requested by Andrew some time ago.

Andrew: you get to test it.