Parse the full ISO 8601 grammar in fromdateiso8601 (#3617)

`fromdateiso8601` was `strptime("%Y-%m-%dT%H:%M:%SZ") | mktime`, which
accepts only one of the many representations ISO 8601 permits. strptime
cannot be extended to cover the rest portably, since it has no specifier
for fractional seconds and glibc discards `%G` and `%V`.

Replace it with a parser in src/iso8601.c that handles basic and extended
representations, ordinal and week dates, fractional seconds, offsets and
date-only input. It uses integer arithmetic instead of `time_t`, so the
results no longer depend on the platform.

This is stricter than before. Impossible dates like `2015-02-30`, which
`mktime` used to normalize into March, and unpadded components like
`2015-3-5T6:00:00Z` are now errors. So is any trailing input, including
whitespace. Leading whitespace is rejected too; before, glibc skipped it
and macOS did not. A leap second now maps to the second before it rather
than the next minute. The error message has changed, and the result can
now be non-integer.

docs/content/manual/dev/manual.yml

@@ -2250,13 +2250,45 @@ sections:
 
           The `fromdateiso8601` builtin parses datetimes in the ISO 8601
           format to a number of seconds since the Unix epoch
-          (1970-01-01T00:00:00Z).  The `todateiso8601` builtin does the
-          inverse.
+          (1970-01-01T00:00:00Z).  The `todateiso8601` builtin formats a
+          number of seconds as `"%Y-%m-%dT%H:%M:%SZ"`.
 
-          The `fromdate` builtin parses datetime strings.  Currently
-          `fromdate` only supports ISO 8601 datetime strings, but in the
-          future it will attempt to parse datetime strings in more
-          formats.
+          `fromdateiso8601` accepts a complete date, optionally followed by
+          a time of day, written in either the extended representation, which
+          separates the components, or the basic representation, which does
+          not.  The two cannot be mixed within one datetime.  The date may be
+          a calendar date (`2015-03-05`, `20150305`), an ordinal date
+          (`2015-064`, `2015064`), or a week date (`2015-W10-4`, `2015W104`);
+          all three denote the same day.  Years outside the range 0000 to 9999
+          are written with a sign and six digits, as in `+012015-03-05`.
+
+          The time of day may omit its seconds, or its minutes and seconds,
+          as in `2015-03-05T23:51Z` and `2015-03-05T23Z`, or be left out
+          altogether, in which case the value is midnight UTC.  It may carry a
+          fractional part after a `.` or a `,`, which belongs to the lowest
+          component present, so `23,5` is half past 23.  It may be followed
+          by an offset from UTC written as `Z`, `+hh`, `+hh:mm` or `+hhmm`;
+          a datetime with no offset is read as UTC.  A time of `24:00:00`
+          denotes midnight of the following day, and a leap second is read as
+          the second before it, since jq counts every day as 86400 seconds.
+
+          The `W`, `T` and `Z` designators may also be written in lower
+          case, which ISO 8601 permits where upper case is not available.
+          RFC 3339 additionally allows the date and the time to be separated
+          by a space instead of a `T` "for the sake of readability", which jq
+          accepts too.  The offset may use the other representation's
+          separator style, as in `2015-03-05T23:51:47+0900`, although
+          ISO 8601 does not allow the basic and extended representations
+          to be mixed.
+
+          A complete date is required, so `2015` and `2015-03` are errors
+          even though ISO 8601 defines them as dates, and `T23:51:47` is an
+          error because a time of day on its own is not a point in time.
+          Dates that do not exist, such as `2015-02-30` or `2016-W53-1`, are
+          errors too, as is `2015-03-05Z`, since a zone designator belongs to
+          the time of day.
+
+          The `fromdate` builtin is an alias for `fromdateiso8601`.
 
           The `todate` builtin is an alias for `todateiso8601`.
 
@@ -2296,8 +2328,11 @@ sections:
           the local timezone setting.
 
           The format strings for `strptime` and `strftime` are described
-          in typical C library documentation.  The format string for ISO
-          8601 datetime is `"%Y-%m-%dT%H:%M:%SZ"`.
+          in typical C library documentation.  `"%Y-%m-%dT%H:%M:%SZ"` is
+          the format that `todateiso8601` produces; it is only one of the
+          ISO 8601 representations that `fromdateiso8601` accepts, and
+          `strptime` cannot parse the others -- there is no portable format
+          specifier for week dates or for fractional seconds.
 
           jq may not support some or all of this date functionality on
           some systems. In particular, the `%u` and `%j` specifiers for
@@ -2308,6 +2343,14 @@ sections:
             input: '"2015-03-05T23:51:47Z"'
             output: ['1425599507']
 
+          - program: 'fromdateiso8601'
+            input: '"2015-03-06T08:51:47.25+09:00"'
+            output: ['1425599507.25']
+
+          - program: 'map(fromdateiso8601)'
+            input: '["20150305T235147Z","2015-064T23:51:47Z","2015-W10-4T23:51:47Z"]'
+            output: ['[1425599507,1425599507,1425599507]']
+
           - program: 'strptime("%Y-%m-%dT%H:%M:%SZ")'
             input: '"2015-03-05T23:51:47Z"'
             output: ['[2015,2,5,23,51,47,4,63]']