dyn-slice 3.2.2

&dyn [Trait] implementation, inspired by a Reddit thread.
Documentation 
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error[E0502]: cannot borrow `slice_1` as mutable because it is also borrowed as immutable
 --> compile_fail_tests/mut_lock_3.rs
  |
  |     let element = slice_1.get(1).unwrap();
  |                   -------------- immutable borrow occurs here
  |
  |     let _ = slice_1.slice_mut(2..4).unwrap();
  |             ^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
  |
  |     let _ = element.borrow();
  |             ---------------- immutable borrow later used here