Standardize terminology for pg_statistic_ext entries.

Standardize terminology for pg_statistic_ext entries.

Consistently refer to such an entry as a "statistics object", not just
"statistics" or "extended statistics".  Previously we had a mismash of
terms, accompanied by utter confusion as to whether the term was
singular or plural.  That's not only grating (at least to the ear of
a native English speaker) but could be outright misleading, eg in error
messages that seemed to be referring to multiple objects where only one
could be meant.

This commit fixes the code and a lot of comments (though I may have
missed a few).  I also renamed two new SQL functions,
pg_get_statisticsextdef -> pg_get_statisticsobjdef
pg_statistic_ext_is_visible -> pg_statistics_obj_is_visible
to conform better with this terminology.

I have not touched the SGML docs other than fixing those function
names; the docs certainly need work but it seems like a separable task.

Discussion: https://postgr.es/m/22676.1494557205@sss.pgh.pa.us