Use serialization for columns of type "list"

[krlmlr]

in join (#2194) and distinct (#2222) operations.

We should be able to serialize all elements of a list efficiently by calling R_Serialize() for each. A specialized version of JoinVisitorImpl and VectorVisitor would then operate on the serializations. Perhaps hashing the serialization will be good enough.

[romainfrancois]

We can look at what R does in unique.c about lists.

[romainfrancois]

Pretty sure this is a vctrs thing now.

@hadley I'm not sure yet at what level we'll use vctrs but if at low level then an api c function for hash_scalar would help.

[romainfrancois]

> vctrs:::vec_hash(list(1:2,c(1L,2L)))
[1] "656ac61" "656ac61"

[hadley]

I think we can probably close this issue; once dplyr can use vctrs, it can use the standard hashing approach defined there.

[krlmlr]

Let's keep this open to remind us that we still need to enable it here once we have it in vctrs.