use rust 2018 syntax, remove unused headers

yaserde_derive/src/de/expand_enum.rs

@@ -1,5 +1,5 @@
-use attribute::*;
-use field_type::*;
+use crate::attribute::*;
+use crate::field_type::*;
 use proc_macro2::TokenStream;
 use std::collections::BTreeMap;
 use syn::spanned::Spanned;

yaserde_derive/src/de/expand_struct.rs

@@ -1,6 +1,6 @@
-use attribute::*;
-use de::build_default_value::build_default_value;
-use field_type::*;
+use crate::attribute::*;
+use crate::de::build_default_value::build_default_value;
+use crate::field_type::*;
 use proc_macro2::{Span, TokenStream};
 use std::collections::BTreeMap;
 use syn::spanned::Spanned;

yaserde_derive/src/de/mod.rs

@@ -2,7 +2,7 @@ pub mod build_default_value;
 pub mod expand_enum;
 pub mod expand_struct;
 
-use attribute;
+use crate::attribute;
 use proc_macro2::TokenStream;
 use syn;
 use syn::Ident;