feat: Implement extend from slice on SoAVec

[Reinsdyret]

extend_from_slice for SoAVec
uses T::TupleReps::write to write each element from slice into self.buf and then update length.

Closes #25

[Reinsdyret]

Unsure about the runtime here if its O(slice.len()) or O(slice.len() * num_fields) depends on into tuple i think

[aapoalas]

Unsure about the runtime here if its O(slice.len()) or O(slice.len() * num_fields) depends on into tuple i think

The tuple is (using the derive macro, and under any reasonable custom implementation) determined by the number of fields in the original struct. O(slice.len() * num_fields) is correct (although someone might argue that that's the case for a normal Vec as well; they'd be both right and wrong).

[aapoalas]

LGTM, and praise for a nice set of tests as well! <3