Fix ArrayString to implement .push(char) faster

Previously we used formatting, which is a virtual call and quite the
detour. Now copy the utf-8 encoding code from Rust (thank you Alex
Crichton) and use that.

src/lib.rs

@@ -55,6 +55,7 @@ use nodrop::NoDrop;
 
 mod array;
 mod array_string;
+mod char_ext;
 
 pub use array::Array;
 pub use odds::IndexRange as RangeArgument;