ASSFade:new(): don't touch original args.raw table passed as argument
prevents defaults from being overwritten
skip tag removal when the section doesn't have any tags
fixes an error being thrown because the start and end indexes were out of bounds
ASSDrawing: fix a missing table index when constructed from a raw table; wrap all default (raw) tag values in a table
support raw string input in the drawing constructor
avoids useless table creation
add "<=" metatable event to ASSNumber; deduplicate stuff a little
move metamethods to the ASS classes (instead of their metatables) so that they are available to objects of this class;
this is product of my stupidity and a partial revert of @bd5f163c67492d3d45e6d069899d4adcb1aaa597; separate metamethod handling is no longer required, because the methods are regular class members