Rename `vec_unchop()` to `list_unchop()`

[DavisVaughan]

Closes #1209

I went ahead and soft deprecated vec_unchop() as well. I think it is mostly us that use it, so having a soft deprecation warning show up in tests is probably a pretty low pain way to get us to move to list_unchop()

[lionel-]

Since we're now introducing a similar function in purrr for the name is still under discussion, maybe we should wait until this is resolved? Should we discuss the naming at the meeting?

[DavisVaughan]

I was under the impression that we'd have to do this renaming no matter what because vec_unchop() has the wrong prefix, but generally we want something that is symmetric with vec_chop() (so we'd keep the unchop suffix)

[DavisVaughan]

@lionel- do you want to do a review of this since we decided to push forward with this name change?