Add method tm_format to class Tm.

[cspiel]

Previous Class Tm

Class Tm provides all components of the "usual" broken-down date-time,
but it is tedious to produce a nicely formatted date-time tag for example for
a subdirectory. The author of this pull request ended up writing a monstrosity like
this for a RFC-3339 formatted date-time:

date_rfc_3339(~use_localtime, ?timespec = $'seconds') =
        pad02(a_number) =
                if $(le $(a_number), 9)
                    return $'0'$(a_number)
                else
                    return $(a_number)

        localtime = $(localtime $(gettimeofday))
        gmtime = $(gmtime $(gettimeofday))
        if $(use_localtime)
            now = $(localtime)
            offset_hour = $(pad02 $(sub $(localtime.tm_hour), $(gmtime.tm_hour)))
            offset_minute = $(pad02 $(sub $(localtime.tm_min), $(gmtime.tm_min)))
            export
        else
            now = $(gmtime)
            offset_hour = $'00'
            offset_minute = $'00'
            export

        year = $(add 1900, $(now.tm_year))
        month = $(pad02 $(add 1, $(now.tm_mon)))
        day_of_month = $(pad02 $(add 1, $(now.tm_mday)))

        hour = $(pad02 $(now.tm_hour))
        minute = $(pad02 $(now.tm_min))
        second = $(pad02 $(now.tm_sec))

        switch $(timespec)
        case date
            return $(concat $'-', $(year) $(month) $(day_of_month))
        case seconds
            return $(concat $'-', $(year) $(month) $(day_of_month)) $(concat $':', $(hour) $(minute) $(second))+$(offset_hour):$(offset_minute)
        default
            raise $'unknown timespec value'

New Method tm_format

This p/r adds a method called tm_format to the Tm class that
works like strftime(3) as specified by POSIX plus some GLibc extensions
minus all locale functionality. The above RFC-3339 example now boils
down to more or less

now = $(gettimeofday)
datetime = $(gmtime $(now))
println($(datetime.tm_format $'%Y-%m-%dT%H:%M:%S%z'))
println($(datetime.tm_format $'%FT%T%z')) # use predefined conversions

Implementation

I have attached the method as a Omake_value_type.ValPrim in Omake_builtin_sys.tm_object.
My attempts to use Omake_value_type.ValFun failed because the statements associated
with ValFun get executed on object construction time; they are not delayed until the method
gets called. Maybe someone with a more profound insight into the IR can suggest a better
Omake_value_type for the method.