parse and create strings in the rfc3339 time format
part of sph-lib
rfc3339 is a subset of iso8601 and is used for example in the atom syndication format.
this is a comprehensive implementation that uses a parsing expression grammar.
(sph time rfc3339)
a [offset seconds-fraction] ->
integer:posix-time -> string
string -> integer:seconds:posix-time
does not include fractional seconds; see rfc3339 ->
seconds-and-fraction for that
string -> (integer:tai-utc-unix-seconds . integer:seconds-fraction)