Merge #2988

2988: Remove const_fn_trait_bound feature and update nightly (Mar 2022) r=lschuermann a=bradjc

### Pull Request Overview

rust-lang/rust#93827 was merged, so there should soon be a nightly that allows us to remove `const_fn_trait_bound` feature. It seems like some of the tools we use aren't working right now, but this PR is a placeholder for when we can update to a new enough nightly.


After this, our list of features looks like:

```
arch/cortex-m/src/lib.rs:#![feature(asm_sym)]
arch/cortex-m/src/lib.rs:#![feature(naked_functions)]
arch/cortex-m0/src/lib.rs:#![feature(asm_sym, naked_functions)]
arch/cortex-m0p/src/lib.rs:#![feature(naked_functions)]
arch/rv32i/src/lib.rs:#![feature(asm_sym, naked_functions)]
boards/esp32-c3-devkitM-1/src/main.rs:#![feature(custom_test_frameworks)]
boards/nano_rp2040_connect/src/main.rs:#![feature(naked_functions)]
boards/opentitan/src/main.rs:#![feature(custom_test_frameworks)]
boards/pico_explorer_base/src/main.rs:#![feature(naked_functions)]
boards/raspberry_pi_pico/src/main.rs:#![feature(naked_functions)]
chips/earlgrey/src/lib.rs:#![feature(naked_functions)]
chips/esp32-c3/src/lib.rs:#![feature(naked_functions)]
chips/litex_vexriscv/src/lib.rs:#![feature(asm_const)]
chips/lowrisc/src/lib.rs:#![feature(const_mut_refs)]
chips/swervolf-eh1/src/lib.rs:#![feature(naked_functions)]
doc/courses/rustconf/application.md:#![feature(alloc)]
kernel/src/lib.rs:#![feature(core_intrinsics)]
libraries/riscv-csr/src/lib.rs:#![feature(asm_const)]
libraries/tock-cells/src/lib.rs:#![feature(const_mut_refs)]
```

### Testing Strategy

travis


### TODO or Help Wanted

Need a version of nightly with all valid tools.


### Documentation Updated

- [x] Updated the relevant files in `/docs`, or no updates are required.

### Formatting

- [x] Ran `make prepush`.


Co-authored-by: Brad Campbell <[email protected]>

Author: bors[bot]

libraries/tock-register-interface/src/lib.rs

@@ -53,7 +53,6 @@
 //! ------
 //! - Shane Leonard <[email protected]>
 
-#![feature(const_fn_trait_bound)]
 #![no_std]
 // If we don't build any actual register types, we don't need unsafe
 // code in this crate