Rename feature to match RFC filename
-- Feature Name: async_rust_constraints
+- Feature Name: async_rust_in_zebra
- Start Date: 2021-03-30
- Design PR: [ZcashFoundation/zebra#1965](https://github.com/ZcashFoundation/zebra/pull/1965)
- Zebra Issue: [ZcashFoundation/zebra#1593](https://github.com/ZcashFoundation/zebra/issues/1593)
