Remove most extern crate statements from yaserde_derive

yaserde_derive is an edition 2018 crate, so these are not necessary.

yaserde_derive/src/common/attribute.rs

@@ -1,4 +1,5 @@
 use proc_macro2::{token_stream::IntoIter, Delimiter, Ident, TokenStream, TokenTree};
+use quote::quote;
 use std::collections::BTreeMap;
 use syn::Attribute;
 

yaserde_derive/src/common/field.rs

@@ -2,6 +2,7 @@ use crate::common::attribute::YaSerdeAttribute;
 use heck::CamelCase;
 use proc_macro2::Span;
 use proc_macro2::{Ident, TokenStream};
+use quote::{format_ident, quote};
 use std::fmt;
 use syn::ext::IdentExt;
 use syn::spanned::Spanned;

yaserde_derive/src/de/build_default_value.rs

@@ -1,5 +1,6 @@
 use crate::common::YaSerdeField;
 use proc_macro2::TokenStream;
+use quote::quote;
 
 pub fn build_default_value(
   field: &YaSerdeField,

yaserde_derive/src/de/expand_enum.rs

@@ -1,5 +1,6 @@
 use crate::common::{Field, YaSerdeAttribute, YaSerdeField};
 use proc_macro2::TokenStream;
+use quote::quote;
 use syn::{DataEnum, Fields, Ident};
 
 pub fn parse(

yaserde_derive/src/de/expand_struct.rs

@@ -2,6 +2,7 @@ use crate::common::{Field, YaSerdeAttribute, YaSerdeField};
 use crate::de::build_default_value::build_default_value;
 use heck::CamelCase;
 use proc_macro2::{Span, TokenStream};
+use quote::quote;
 use syn::{DataStruct, Ident};
 
 pub fn parse(

yaserde_derive/src/de/mod.rs

@@ -4,6 +4,7 @@ pub mod expand_struct;
 
 use crate::common::YaSerdeAttribute;
 use proc_macro2::TokenStream;
+use quote::quote;
 use syn::Ident;
 
 pub fn expand_derive_deserialize(ast: &syn::DeriveInput) -> Result<TokenStream, String> {

yaserde_derive/src/lib.rs

@@ -1,10 +1,7 @@
 #![recursion_limit = "256"]
 
+// Required for Rust < 1.42
 extern crate proc_macro;
-extern crate proc_macro2;
-#[macro_use]
-extern crate quote;
-extern crate syn;
 
 mod common;
 mod de;

yaserde_derive/src/ser/element.rs

@@ -1,5 +1,6 @@
 use crate::common::YaSerdeField;
 use proc_macro2::{Ident, TokenStream};
+use quote::quote;
 
 pub fn enclose_formatted_characters(label: &Ident, label_name: String) -> TokenStream {
   enclose_xml_event(label_name, quote!(format!("{}", &self.#label)))

yaserde_derive/src/ser/expand_enum.rs

@@ -1,6 +1,7 @@
 use crate::common::{Field, YaSerdeAttribute, YaSerdeField};
 use crate::ser::{implement_serializer::implement_serializer, label::build_label_name};
 use proc_macro2::TokenStream;
+use quote::quote;
 use syn::DataEnum;
 use syn::Fields;
 use syn::Ident;

yaserde_derive/src/ser/expand_struct.rs

@@ -2,6 +2,7 @@ use crate::common::{Field, YaSerdeAttribute, YaSerdeField};
 
 use crate::ser::{element::*, implement_serializer::implement_serializer};
 use proc_macro2::TokenStream;
+use quote::quote;
 use syn::DataStruct;
 use syn::Ident;
 

yaserde_derive/src/ser/implement_serializer.rs

@@ -2,6 +2,7 @@ use crate::common::YaSerdeAttribute;
 use crate::ser::namespace::generate_namespaces_definition;
 use proc_macro2::Ident;
 use proc_macro2::TokenStream;
+use quote::quote;
 
 pub fn implement_serializer(
   name: &Ident,

yaserde_derive/src/ser/mod.rs

@@ -7,6 +7,7 @@ pub mod namespace;
 
 use crate::common::YaSerdeAttribute;
 use proc_macro2::TokenStream;
+use quote::quote;
 use syn::Ident;
 
 pub fn expand_derive_serialize(ast: &syn::DeriveInput) -> Result<TokenStream, String> {

yaserde_derive/src/ser/namespace.rs

@@ -1,5 +1,6 @@
 use crate::common::YaSerdeAttribute;
 use proc_macro2::TokenStream;
+use quote::quote;
 
 pub fn generate_namespaces_definition(attributes: &YaSerdeAttribute) -> TokenStream {
   attributes