Increase default number of steps of Rust example to 10^9
Increase default number of steps of Rust example to 10^9

This matches default value of other compiled languages (C/C+, Fortran, Julia).

UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples,UCL-RITS/pi_examples