Add string_to_table() function.

Add string_to_table() function.

This splits a string at occurrences of a delimiter.  It is exactly like
string_to_array() except for producing a set of values instead of an
array of values.  Thus, the relationship of these two functions is
the same as between regexp_split_to_table() and regexp_split_to_array().

Although the same results could be had from unnest(string_to_array()),
this is somewhat faster than that, and anyway it seems reasonable to
have it for symmetry with the regexp functions.

Pavel Stehule, reviewed by Peter Smith

Discussion: https://postgr.es/m/CAFj8pRD8HOpjq2TqeTBhSo_QkzjLOhXzGCpKJ4nCs7Y9SQkuPw@mail.gmail.com