Implement parse_datetime() function

This commit adds parse_datetime() function, which implements datetime
parsing with extended features demanded by upcoming jsonpath .datetime()
method:

 * Dynamic type identification based on template string,
 * Support for standard-conforming 'strict' mode,
 * Timezone offset is returned as separate value.

Extracted from original patch by Nikita Glukhov, Teodor Sigaev, Oleg Bartunov.
Revised by me.

Discussion: https://postgr.es/m/fcc6fc6a-b497-f39a-923d-aa34d0c588e8%402ndQuadrant.com
Discussion: https://postgr.es/m/CAPpHfdsZgYEra_PeCLGNoXOWYx6iU-S3wF8aX0ObQUcZU%2B4XTw%40mail.gmail.com
Author: Nikita Glukhov, Teodor Sigaev, Oleg Bartunov, Alexander Korotkov
Reviewed-by: Anastasia Lubennikova, Peter Eisentraut

src/backend/utils/adt/date.c

@@ -41,11 +41,6 @@
 #endif
 
 
-static int	tm2time(struct pg_tm *tm, fsec_t fsec, TimeADT *result);
-static int	tm2timetz(struct pg_tm *tm, fsec_t fsec, int tz, TimeTzADT *result);
-static void AdjustTimeForTypmod(TimeADT *time, int32 typmod);
-
-
 /* common code for timetypmodin and timetztypmodin */
 static int32
 anytime_typmodin(bool istz, ArrayType *ta)
@@ -1203,7 +1198,7 @@ time_in(PG_FUNCTION_ARGS)
 /* tm2time()
  * Convert a tm structure to a time data type.
  */
-static int
+int
 tm2time(struct pg_tm *tm, fsec_t fsec, TimeADT *result)
 {
 	*result = ((((tm->tm_hour * MINS_PER_HOUR + tm->tm_min) * SECS_PER_MINUTE) + tm->tm_sec)
@@ -1379,7 +1374,7 @@ time_scale(PG_FUNCTION_ARGS)
  * have a fundamental tie together but rather a coincidence of
  * implementation. - thomas
  */
-static void
+void
 AdjustTimeForTypmod(TimeADT *time, int32 typmod)
 {
 	static const int64 TimeScales[MAX_TIME_PRECISION + 1] = {
@@ -1957,7 +1952,7 @@ time_part(PG_FUNCTION_ARGS)
 /* tm2timetz()
  * Convert a tm structure to a time data type.
  */
-static int
+int
 tm2timetz(struct pg_tm *tm, fsec_t fsec, int tz, TimeTzADT *result)
 {
 	result->time = ((((tm->tm_hour * MINS_PER_HOUR + tm->tm_min) * SECS_PER_MINUTE) + tm->tm_sec) *