Add array_fill() to create arrays initialized with a value.

doc/src/sgml/func.sgml

src/backend/utils/adt/arrayfuncs.c

src/include/catalog/catversion.h

src/include/catalog/pg_proc.h

src/include/utils/array.h

src/test/regress/expected/arrays.out

src/test/regress/sql/arrays.sql