Use absolute paths in generated code

Closes #170

maud/tests/absolute_paths.rs

@@ -0,0 +1,13 @@
+#![feature(proc_macro_hygiene)]
+
+// Make sure `std` is available but the prelude isn't
+#![no_std]
+extern crate std;
+
+use maud::html;
+
+#[test]
+fn issue_170() {
+    let number = 42;
+    let _ = html! { (number) };
+}

maud_macros/src/generate.rs

@@ -84,7 +84,7 @@ impl Generator {
         quote!({
             // Create a local trait alias so that autoref works
             trait Render: maud::Render {
-                fn __maud_render_to(&self, output_ident: &mut String) {
+                fn __maud_render_to(&self, output_ident: &mut ::std::string::String) {
                     maud::Render::render_to(self, output_ident);
                 }
             }

maud_macros/src/lib.rs

@@ -46,7 +46,7 @@ fn expand(input: TokenStream) -> TokenStream {
     let stmts = generate::generate(markups, output_ident.clone());
     quote!({
         extern crate maud;
-        let mut $output_ident = String::with_capacity($size_hint);
+        let mut $output_ident = ::std::string::String::with_capacity($size_hint);
         $stmts
         maud::PreEscaped($output_ident)
     })