Std.parseFloat macro failure

Stumbled across this while making changes to Serializer in #2556

Seems macro doesn't parse the "+" correctly after the exponent symbol.

```
//Prints "-1.#IND" for macro and "10" for neko
trace(Std.parseFloat("1e+1"));
```

This is a problem because Std.string() for macro generates a "+" for large numbers.
