Suppress transmute_ptr_to_ptr pedantic clippy lint in generated code

[dtolnay]

warning: transmute from a reference to a reference
  --> tests/test_custom.rs:11:39
   |
11 |         pub fn new(s: &str) -> &Custom;
   |                                       ^ help: try: `&*(s as *const str as *const forbid_unsafe::Custom)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_ptr_to_ptr
   = note: `-W clippy::transmute-ptr-to-ptr` implied by `-W clippy::pedantic`
   = help: to override `-W clippy::pedantic` add `#[allow(clippy::transmute_ptr_to_ptr)]`