title: How to format output to a byte array with no_std and no allocator?
body: <p>I want to do something like:</p>
  
  <pre><code>let x = 123;
  let mut buf = [0 as u8; 20];
  format_to!(x --&gt; buf);
  assert_eq!(&amp;buf[..3], &amp;b"123"[..]);
  </code></pre>
  
  <p>With <code>#![no_std]</code> and without any memory allocator.</p>
  
  <p>As I understand, there is an implementation of <code>core::fmt::Display</code> for <code>u64</code>, and I want to use it if possible.</p>
  
  <p>In other words, I want to do something like <code>format!(...)</code>, but without a memory allocator. How can I do this?</p>