Conditional schedules

• Add if statements to the scheduling language.
• Add a feature(...) function that can be used to test for whether a feature is enabled
  • For this to work, the scheduler needs to be run from a Cargo build script since it relies on testing environment variables (as described here).
• Modified matmul to use a single schedule testing for the "cuda" feature

juno_scheduler/src/pm.rs

@@ -1470,7 +1470,7 @@ fn interp_expr(
             };
             // To test for features, the scheduler needs to be invoked from a build script so that
             // Cargo provides the enabled features via environment variables
-            let key = val.to_uppercase().replace("-", "_");
+            let key = "CARGO_FEATURE_".to_string() + &val.to_uppercase().replace("-", "_");
             Ok((
                 Value::Boolean {
                     val: env::var(key).is_ok(),