Implement helper for data frame subset assignment

[krlmlr]

I have isolated a function in tibble that implements clean subset assignment, without tibble's quirks. This function could move to vctrs, and be reimplemented here in C.

The R code for this function, and helper functions, is here:

https://github.com/tidyverse/tibble/pull/1473/files#diff-99ebfcc4d379143562ab114b81972093b641cd4ce538a91b32393ba872b3a1de

What's a good way to move forward? Should I draft a vctrs PR with tests?

I had attempted a C implementation of one of the helper functions in tidyverse/tibble#1368.