Remove AssStyleStorage's dependencies on other Aegisub classes
Change VariableDataType to an enum class
Change AssParameterClass to an enum class
Move uuencoding logic to libaegisub and add tests
Store ass attachments in the encoded form in memory

This marginally increases memory use, but vastly speeds up pretty much
everything when a file has attachments (other than extracting the
attachments, but that's generally IO-bound anyway).
Don't complain about things in the autoload folder which aren't automation scripts