move mutation management to marauders

[alpaylan]

Marauders is a source level mutation management tool, so I think especially for a long-running version of morris, it would make sense to use something with some persistence. Here's a rundown of how it works with a mock test in the library:

Before (original base file):

  pub fn add(a: i32, b: i32) -> i32 {
      a + b
  }

We get a morris mutation:

  {
    "file_path": "src/lib.rs",
    "line_number": 2,
    "original_line": "    a + b",
    "mutated_line": "    a - b",
    "description": "mock handoff mutation"
  }

After construction (mutation block inserted):

  pub fn add(a: i32, b: i32) -> i32 {
      /*| morris_plan_0001 */
      a + b
      /*|| morris_plan_0001_1 */
      /*|
      a - b
      */
      /* |*/
  }

After set (marauders set --variant morris_plan_0001_1):

  pub fn add(a: i32, b: i32) -> i32 {
      /*| morris_plan_0001 */
      /*|
      a + b
      */
      /*|| morris_plan_0001_1 */
      a - b
      /* |*/
  }

After unset (marauders unset --variant morris_plan_0001_1):

  pub fn add(a: i32, b: i32) -> i32 {
      /*| morris_plan_0001 */
      a + b
      /*|| morris_plan_0001_1 */
      /*|
      a - b
      */
      /* |*/
  }

There's also a feature in Marauders for inserting multiple mutants at the same time (so you don't pay for recompilation many times, it's guarded under syntax-rust-functional so we can also add that.