Implement "rss" version of mr.ash methods

[pcarbo]

A draft implementation can be found here (thanks to Gao, Fabio, and others):
https://github.com/cumc/pecotmr/blob/main/src/mr_ash.h
https://github.com/cumc/pecotmr/blob/main/src/mr_ash.cpp
https://github.com/cumc/pecotmr/blob/main/R/run_mr_ash_rss.R

It would be good to align the mr.ash methods (and function names) to correspond to the "susie_suff_stat" and "susie_rss" functions in the susieR package.