minidom: Make the whole crate almost no_std

Only std::io is still missing, maybe we should consider using a crate
implementing its API on top of no_std?

minidom/src/node.rs

@@ -10,6 +10,7 @@
 
 use crate::element::{Element, ElementBuilder, ItemWriter};
 use crate::error::Result;
+use alloc::string::String;
 
 use rxml::writer::Item;