datetime, lua: interval arithmetics

Add interval arithmetics as date:add{} and date:sub{}, or
overloaded '+' and '-' operators (__add/_sub metamethods).
There is no special processing for timezone specific
daylight saving time changes, which impacts result of arithmetics.
This will be completed after Olsen database support added.

If one needs to control a way arithmetic proceeded for days
of months then there is optional argument to `:add()` or `:sub()`,
which defines how we round day in month after arithmetic
operation:

- 'last' - mode when day snaps to the end of month, if happens:

  -- 29.02.2004 -> 31.03.2004
  dt:add{month = 1, adjust = "last" }

- 'none' only truncation toward the end of month performed
  (**default mode**):

  -- 29.02.* -> 29.03.*
  dt:add{month = 1, adjust = "none" }

- 'excess' - overflow mode, without any snap or truncation to
  the end of month, straight addition of days in month, stopping
  over month boundaries if there is less number of days.

  dt = new{year = 2004, month = 1, day = 31}
  dt:add{month = 1, adjust = 'excess'} -> 02.03.2004

  -- i.e. there were 29 days in February 2004, 2 excessive days
     from original date 31 make result 2nd March 2004

Intervals objects, created via `datetime.interval.new()` call
and objects of the same attributes are equivalent for interval
arithmetics and exchangeable. i.e.

   dt + date.interval.new{year = 1, month = 2}

is the same as
   dt + {year = 1, month = 2}

Follow-up to #5941