Implement `missing = "remove"`

[DavisVaughan]

Closes #1595 Closes #1489

Another fairly complex PR unfortunately, but I've tried to keep each commit self contained again

• I tackled #1489 while I was here, because it was staring me in the face as I modified chr_as_location()

• num_as_location2() and vec_as_location2() did not gain this argument. They have to select exactly 1 element so it doesn't make sense there

I'll mark some other important things inline