SRFI-19: Fix TAI->UTC conversions, leap second handling, etc.

Fixes <https://bugs.gnu.org/21911>.
Fixes <https://bugs.gnu.org/22034>.
Fixes <https://bugs.gnu.org/21902>.
Partially fixes <https://bugs.gnu.org/21904>.
Reported by Zefram <zefram@fysh.org>.

* doc/ref/srfi-modules.texi (SRFI-19 Introduction): Fix the definitions
of Julian Day and Modified Julian Day.  Give the correct full names of
UTC and TAI.
* module/srfi/srfi-19.scm: Import (srfi srfi-1).  Use modern Guile
keyword syntax in the 'define-module' form.
(leap-second-neg-delta): New procedure, derived from a similar procedure
in the latest upstream SRFI-19 reference implementation.
(priv:time-tai->time-utc!, time-tai->julian-day)
(time-monotonic->julian-day): Use 'leap-second-neg-delta'.
(local-tz-offset): Fix comment.
(leap-second?): Remove.
(tai-before-leap-second?): New procedure, derived from upstream SRFI-19.
(time-utc->date): Use 'define*' to handle the optional argument.  Remove
the leap second handling, following upstream SRFI-19.
(time-tai->date): Rewrite in terms of 'time-utc->date'.  Add special
leap second handling, following upstream SRFI-19.
(time-monotonic->date): Rewrite in terms of 'time-tai->date'.
(date->time-tai, date->time-monotonic): Add special leap second
handling, following upstream SRFI-19.
(directives): In the entry for the "~Y" escape in 'date->string', pad
the year field to 4 characters, following upstream SRFI-19.
* test-suite/tests/srfi-19.test: Add tests.

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