Blender Git Loki

bli: generational_arena: add iterator support

Made as a bidirectional iterator since movement can be in both
directions. Random access iterator is not possible since there can be
gaps in between the elements.

Had a very annoying bug- using the iterator with the standard library
would throw an error "`iterator_category` not defined" even though it
was defined. Spent a lot of time to realize that `difference_type`
should also be defined but other types within the iterator class are
optional. For some reason the compiler does not create the
`iterator_trait` correctly if `difference_type` is missing but throws
error about `iterator_category` which is extremely strange and time consuming to debug :(