use usize as underlying type for SpecialCharacterIndices

This allows to use it as slice indice waiting for the `SliceIndex` trait to be stabilized (rust-lang/rust#35729)

src/sys/termios.rs

@@ -401,7 +401,7 @@ libc_enum! {
     }
 }
 
-// TODO: Make this usable directly as a slice index.
+#[repr(usize)]
 libc_enum! {
     /// Indices into the `termios.c_cc` array for special characters.
     #[repr(usize)]