Make a version of char::escape_unicode that doesn't allocate

[catamorphism]

As per FIXME (former XXX) in write_escaped_char in core::repr. I guess escape_unicode could be rewritten to take a closure of type fn(char).

[catamorphism]

Relatedly: the impls of Repr for float, f32, and f64 in the same file also malloc, when they don't have to.

If I am understanding correctly, escape_unicode should be (char, fn(char))? If that is the approach, it also requires changing num:to_str_common and u*:to_str_radix to have a similar signature. That is, it seems as though a lot of the built-in functions that return ~str should instead accept a fn(char) that will be called with each resulting character.

[alexcrichton]

Closed in d3155fa