lint `private_no_mangle_fns`/`private_no_mangle_statics` has been removed

[nanocryk]

When compiling in nightly rustc 1.33.0-nightly (fb86d604b 2018-12-27) or stable rustc 1.31.1 (b6c32da9b 2018-12-18), the following errors appears :

error: lint `private_no_mangle_fns` has been removed: `no longer a warning, #[no_mangle] functions always exported`
   --> src/lib.rs:229:5
    |
229 |     private_no_mangle_fns,
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
note: lint level defined here
   --> src/lib.rs:219:5
    |
219 |     warnings
    |     ^^^^^^^^
    = note: #[forbid(renamed_and_removed_lints)] implied by #[forbid(warnings)]

error: lint `private_no_mangle_statics` has been removed: `no longer a warning, #[no_mangle] statics always exported`
   --> src/lib.rs:230:5
    |
230 |     private_no_mangle_statics,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

error: Could not compile `p2p`.

Removing both deny lints allow to build.