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

requested review from @rarbore2

assigned to @aaronjc4

enabled an automatic merge when all merge checks for 05273c7b pass

merged

mentioned in commit 0717de36