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_samples/matmul/src/gpu.sch

-phi-elim(*);
-
-forkify(*);
-fork-guard-elim(*);
-dce(*);
-
-fixpoint {
-  reduce-slf(*);
-  slf(*);
-  infer-schedules(*);
-}
-fork-coalesce(*);
-infer-schedules(*);
-dce(*);
-rewrite(*);
-fixpoint {
-  simplify-cfg(*);
-  dce(*);
-}
-
-ip-sroa(*);
-sroa(*);
-dce(*);
-
-float-collections(*);
-gcm(*);