A few data points on GNU coreutils.

1. It has a fairly good test suite, that rewrites should leverage. That can be easily done by setting $PATH to prepend the dir of the new tools, and running `make check`

2. To give an indication of the size of coreutils:

$ for r in gnulib coreutils; do (cd $r && git ls-files | tr '\n' '\0' | wc -l --files0-from=- | tail -n1); done

985050 total 243154 total