Allow incomplete_feature for specialization
Specialiazation is now classified as an incomplete feature because of the soundness hole due to specializing on lifetimes.  `min_specialization` is an alternative, but it did not work for our specializations of `fmt::Debug`.
#![deny(warnings)]
// Do not fail the build when feature flags are stabilized on recent nightlies, just warn
#![allow(stable_features)]
+// allow for incomplete `specialization`
+#![allow(incomplete_features)]
// Support backtraces in errors
#![feature(backtrace)]
#![feature(const_fn)]
