Unicode escapes in strings and identifiers

doc/src/sgml/syntax.sgml

src/backend/catalog/sql_features.txt

src/backend/parser/scan.l

src/backend/utils/adt/xml.c

src/backend/utils/mb/wchar.c

src/bin/psql/psqlscan.l

src/include/mb/pg_wchar.h

src/interfaces/ecpg/preproc/pgc.l

src/interfaces/ecpg/preproc/preproc.y

src/interfaces/ecpg/test/ecpg_schedule

src/interfaces/ecpg/test/ecpg_schedule_tcp

src/interfaces/ecpg/test/expected/preproc-strings.c

src/interfaces/ecpg/test/expected/preproc-strings.stderr

src/interfaces/ecpg/test/expected/preproc-strings.stdout

src/interfaces/ecpg/test/preproc/Makefile

src/interfaces/ecpg/test/preproc/strings.pgc

src/test/regress/expected/strings.out

src/test/regress/sql/strings.sql