rand: Rewrite OsRng in Rust for windows
rand: Rewrite OsRng in Rust for windows

This removes even more rust_builtin.c code, and allows us to more gracefully
handle errors (not a process panic, but a task failure).

waynenilsen/rand,bluss/rand,shepmaster/rand,GrahamDennis/rand,arthurprs/rand,huonw/rand,ebfull/rand,bhickey/rand,achanda/rand,retep998/rand