Stop emitting code that triggers the mutable_transmutes lint

[bjorn3]

Transmuting from a reference to a mutable reference is insta-UB: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8da5074ba78a22ba108e198273c547ba

error: Undefined Behavior: writing to alloc1 which is read-only
 --> src/main.rs:5:17
  |
5 |         let _ = core::mem::transmute::<&[u8; 10], &mut [libc::c_char; 10]>(b"\0\0\0\0\0\0\0\0\0\0");
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ writing to alloc1 which is read-only
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
  = note: BACKTRACE:
  = note: inside `main` at src/main.rs:5:17: 5:100

C2rust currently does such transmutes for code like char mode2[10] = ""; where it produces let mut mode2: [libc::c_char; 10] = *core::mem::transmute::<&[u8; 10], &mut [libc::c_char; 10]>(b"\0\0\0\0\0\0\0\0\0\0"); even though let mut mode2: [libc::c_char; 10] = [0; 10]; works just fine.