`#![feature(const_fn)]` has been removed

[YoshieraHuang]

Hi, I tried to compile drone with rustc of version 1.57.0-nightly but it failed, showing:

error[E0557]: feature has been removed
  --> /home/jasper/.cargo/registry/src/github.1485827954.workers.dev-1ecc6299db9ec823/drone-core-0.14.3/src/lib.rs:27:12
   |
27 | #![feature(const_fn)]
   |            ^^^^^^^^ feature has been removed
   |
   = note: split into finer-grained feature gates

As this said, the feature has been removed and should use #![feature(const_mut_refs)] instead.