message: Fix the build by updating `linked_list_allocator` to 0.8.7.
  `linked_list_allocator` requires you to specify the `const_mut_refs` option in order for `Heap::empty` to be `const`. This broke the build of `libtock_core` when `linked_list_allocator` 0.8.7 was pushed to crates.io.
diff: @@ -10,7 +10,12 @@ alloc_init = []
  custom_panic_handler = []
  custom_alloc_error_handler = []
  +[dependencies.linked_list_allocator]
  +default-features = false
  +features = ["const_mut_refs"]
  +optional = true
  +version = "0.8.7"
  +
  [dependencies]
  -linked_list_allocator = { optional = true, version = "0.8.6", default-features = false }
  libtock_codegen = { path = "../codegen" }
  libtock_platform = { path = "platform" }
  