Improve Step::forward/backward for optimization
Improve Step::forward/backward for optimization

The previous definition did not optimize down to a single add operation,
but this version does appear to.

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