[acc-errors] Accumulate errors for Seq and Vector parsers

[bergmark]

Naming has room for improvements, maybe we should even have a module for accumulation related functions, or should we do this in some other way like using a newtype so we can have proper traversable instances (not sure if that'll work out)?

Ping @Lysxia

[bergmark]

I'm not sure when I'll get around to playing more with this, so feel free to steal the commit if anyone wants to keep going before I get the chance.

[bergmark]

I changed this to use an AccParser newtype, much nicer!