Update to edition 2018

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

src/element.rs

@@ -1,5 +1,10 @@
 //! Provides an `Element` type, which represents DOM nodes, and a builder to create them with.
 
+use crate::convert::IntoAttributeValue;
+use crate::error::{Error, Result};
+use crate::namespace_set::NamespaceSet;
+use crate::node::Node;
+
 use std::io:: Write;
 use std::collections::{btree_map, BTreeMap};
 
@@ -7,8 +12,6 @@ use std::str;
 use std::rc::Rc;
 use std::borrow::Cow;
 
-use error::{Error, Result};
-
 use quick_xml::Reader as EventReader;
 use quick_xml::Writer as EventWriter;
 use quick_xml::events::{Event, BytesStart, BytesEnd, BytesDecl};
@@ -19,10 +22,6 @@ use std::str::FromStr;
 
 use std::slice;
 
-use convert::IntoAttributeValue;
-use namespace_set::NamespaceSet;
-use node::Node;
-
 /// helper function to escape a `&[u8]` and replace all
 /// xml special characters (<, >, &, ', ") with their corresponding
 /// xml escaped value.