message: start_prototype: replace hardcoded class IDs with constants, + rustfmt
  Because this file is not built as part of libtock_runtime in CI, its formatting was not checked. I ran rustfmt on the file, which produced a couple extra unrelated changes.
diff: @@ -22,9 +22,15 @@ struct RtHeader {
  bss_start: *mut u8,
  }
  +mod class_id {
  + pub const COMMAND: usize = 2;
  + pub const MEMOP: usize = 5;
  + pub const EXIT: usize = 6;
  +}
  +
  #[link_section = ".start"]
  #[no_mangle]
  -extern fn start_prototype(
  +extern "C" fn start_prototype(
  rt_header: &RtHeader,
  _memory_start: usize,
  _memory_len: usize,
  @@ -44,14 +50,21 @@ extern fn start_prototype(
  // Binary is in an incorrect location: report an error via
  // LowLevelDebug then exit.
  unsafe {
  - TockSyscalls::syscall4::<2>([8 as *mut (), 1 as *mut (), 2 as *mut (), 0 as *mut ()]);
  - TockSyscalls::syscall2::<6>([0 as *mut (), 0 as *mut ()]);
  + TockSyscalls::syscall4::<class_id::COMMAND>([
  + 8 as *mut (),
  + 1 as *mut (),
  + 2 as *mut (),
  + 0 as *mut (),
  + ]);
  + TockSyscalls::syscall2::<class_id::EXIT>([0 as *mut (), 0 as *mut ()]);
  }
  }
  // Set the app break.
  // TODO: Replace with Syscalls::memop_brk() when that is implemented.
  - unsafe { TockSyscalls::syscall2::<5>([0 as *mut (), rt_header.initial_break]); }
  + unsafe {
  + TockSyscalls::syscall2::<class_id::MEMOP>([0 as *mut (), rt_header.initial_break]);
  + }
  // Set the stack pointer.
  unsafe {
  @@ -87,9 +100,11 @@ extern fn start_prototype(
  remaining -= 1;
  }
  - extern {
  + extern "C" {
  fn rust_start() -> !;
  }
  - unsafe { rust_start(); }
  + unsafe {
  + rust_start();
  + }
  }
  