Combine data frames in parallel in `vec_c()` and `vec_rbind()`

[lionel-]

• Save column pointers as in the hashing code, for the output and for all the inputs.

• Once we have only array pointers we can assign in parallel.

• List-cols involve the R API and so must be assigned separately.

• df-cols could in principle be assigned in parallel too, but this would be a bit more complex.

Should make a big improvement when recombining data frames with vec_unchop().

Technical challenge: Figure out recursive proxying and restoration. Will require keeping a tree of restoration targets. Also need to think through interaction with method composition of proxy and restore.

[lionel-]

According to benchmarks from @romainfrancois the combination step isn't the main bottleneck.