Can not be compatible with versions of rustc >= 1.56.1

[JimChenWYU]

https://ci.appveyor.com/project/Peternator7/strum/builds/50627236

error[E0658]: deriving `Default` on enums is experimental
   --> C:\Users\appveyor\.cargo\registry\src\github.com-1ecc6299db9ec823\unicode-width-0.1.13\src\lib.rs:204:30
    |
204 | #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
    |                              ^^^^^^^
    |
    = note: see issue #86985 <https://github.com/rust-lang/rust/issues/86985> for more information
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0658`.
error: could not compile `unicode-width` due to previous error

I found strum depend on clap:=2.33.0, but clap:=2.33.0 depend on unicode-width:^0.1.4

unicode-width has breaking change in latest version.

[Peternator7]

Fixed by #384