Fix bugs found by 2019-09-29 regexp scanner

Fix bugs found by 2019-09-29 regexp scanner

Problems reported by Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2019-09/threads.html
* lisp/calendar/iso8601.el (iso8601--year-match)
(iso8601--full-date-match, iso8601--without-day-match)
(iso8601--week-date-match, iso8601--ordinal-date-match)
(iso8601--zone-match):
* lisp/textmodes/rst.el (rst-re-alist-def):
Put ‘-’ at the end of bracketed ranges, following the style
suggestion in the Elisp manual.
(iso8601--time-match): Use \([0-9]*\) instead of \([0-9]+\)?
to pacify the regexp scanner.
(iso8601-parse-time): Adjust accordingly.
* lisp/language/burmese.el (burmese-composable-pattern):
* lisp/language/indian.el (devanagari-composable-pattern)
(bengali-composable-pattern, gurmukhi-composable-pattern)
(gujarati-composable-pattern, oriya-composable-pattern)
(telugu-composable-pattern, kannada-composable-pattern)
(malayalam-composable-pattern):
Prefer [ab] to [a-b] when the characters differ by 1,
to pacify the regexp scanner.
* lisp/language/burmese.el (burmese-composable-pattern):
Fix missing-‘\u’ typos.
* lisp/language/indian.el (gurmukhi-composable-pattern):
Fix missing-‘\’ typo.
* lisp/language/tibetan.el (tibetan-regexp):
Quote ‘+’ in regexp to pacify the regexp scanner.  Simplify.
* lisp/textmodes/rst.el (rst-re-alist-def): Fix ‘[]-'...]’
typo by putting the ‘-’ at end of the bracketed expression.