change prolog_parse_rebis to use 2018 edition rust

so all crates use the same edition

crates/prolog_parser/src/lib.rs

@@ -1,14 +1,14 @@
-extern crate lexical;
-extern crate ordered_float;
-#[cfg(feature = "rug")]
-extern crate rug;
 #[cfg(feature = "num-rug-adapter")]
-extern crate num_rug_adapter as rug;
-extern crate unicode_reader;
+use num_rug_adapter as rug;
+#[cfg(feature = "rug")]
+use rug;
 
-#[macro_use] pub mod tabled_rc;
-#[macro_use] pub mod ast;
-#[macro_use] pub mod macros;
+#[macro_use]
+pub mod tabled_rc;
+#[macro_use]
+pub mod ast;
+#[macro_use]
+pub mod macros;
 pub mod parser;
 pub mod put_back_n;