Cleanup and fix warnings on example

Cleanup of some warnings in example

$ cargo +nightly build --release --target wasm32-unknown-unknown

   Compiling wee_alloc_example v0.1.0 (/home/sharks/source/wee_alloc/example)
warning: unnecessary `unsafe` block
  --> example/src/lib.rs:24:5
   |
24 |     unsafe {
   |     ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default

warning: unnecessary `unsafe` block
  --> example/src/lib.rs:34:5
   |
34 |     unsafe {
   |     ^^^^^^ unnecessary `unsafe` block

warning: `extern` fn uses type `core::alloc::Layout`, which is not FFI-safe
  --> example/src/lib.rs:33:26
   |
33 | pub extern "C" fn oom(_: ::core::alloc::Layout) -> ! {
   |                          ^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: `#[warn(improper_ctypes_definitions)]` on by default
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

warning: 3 warnings emitted

    Finished release [optimized + debuginfo] target(s) in 0.21s

rustwasm / wee_alloc

Cleanup and fix warnings on example #89