ErlNifCharEncoding's DUMMY variant may be no longer necessary

[koba-e964]

enum ErlNifCharEncoding has the DUMMY variant to prevent a "univariant enum" compile error from happening.

rustler/rustler_sys/src/rustler_sys_api.rs

Lines 185 to 191 in b1e09e2

	/// See [ErlNifCharEncoding](http://www.erlang.org/doc/man/erl_nif.html#ErlNifCharEncoding) in the Erlang docs.
	#[derive(Debug, Copy, Clone)]
	#[repr(C)]
	pub enum ErlNifCharEncoding {
	ERL_NIF_LATIN1 = 1,
	DUMMY = 999, // prevents "univariant enum" compile error
	}

But this enum without this DUMMY variant no longer emits a compile error with the latest stable release of rustc.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ee80075ae7081fe3e634f8cb402e45df

I believe that this "univariant enum" issue was fixed in rust-lang/rust#33355, which landed in rustc 1.10.0 (2016-07-07). Maybe we can mark the DUMMY variant as deprecated and remove it in the next release.

[filmor]

This was fixed in #390, thank you for the hint :)