RustWrapper: just use the *AtIndex funcs directly
RustWrapper: just use the *AtIndex funcs directly

Otherwise we're kind of reimplementing the inverse of the well-named
methods, and that's not a direction we want to go.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust