FEAT: Remove odds dependency in arrayvec

Copy the encode_utf8 function from odds. std encode_utf8 requires Rust
1.15 and has a different signature, this one seems to fit us better.

src/char.rs

@@ -0,0 +1,54 @@
+// Copyright 2012-2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+//
+// Original authors: alexchrichton, bluss
+
+// UTF-8 ranges and tags for encoding characters
+const TAG_CONT: u8    = 0b1000_0000;
+const TAG_TWO_B: u8   = 0b1100_0000;
+const TAG_THREE_B: u8 = 0b1110_0000;
+const TAG_FOUR_B: u8  = 0b1111_0000;
+const MAX_ONE_B: u32   =     0x80;
+const MAX_TWO_B: u32   =    0x800;
+const MAX_THREE_B: u32 =  0x10000;
+
+/// Placeholder
+pub struct EncodeUtf8Error;
+
+/// Encode a char into buf using UTF-8.
+///
+/// On success, return the byte length of the encoding (1, 2, 3 or 4).<br>
+/// On error, return `EncodeUtf8Error` if the buffer was too short for the char.
+#[inline]
+pub fn encode_utf8(ch: char, buf: &mut [u8]) -> Result<usize, EncodeUtf8Error>
+{
+    let code = ch as u32;
+    if code < MAX_ONE_B && buf.len() >= 1 {
+        buf[0] = code as u8;
+        return Ok(1);
+    } else if code < MAX_TWO_B && buf.len() >= 2 {
+        buf[0] = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
+        buf[1] = (code & 0x3F) as u8 | TAG_CONT;
+        return Ok(2);
+    } else if code < MAX_THREE_B && buf.len() >= 3 {
+        buf[0] = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
+        buf[1] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
+        buf[2] = (code & 0x3F) as u8 | TAG_CONT;
+        return Ok(3);
+    } else if buf.len() >= 4 {
+        buf[0] = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
+        buf[1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
+        buf[2] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
+        buf[3] = (code & 0x3F) as u8 | TAG_CONT;
+        return Ok(4);
+    };
+    Err(EncodeUtf8Error)
+}
+