std.format with wstring and dstring

[Robert Schadek]

I spend one day at dconf this year and removed wstring and 
dstring support from
std.format to improve compile speed. std.format.FormatSpec is no 
longer a template
on the character type, and the bitfield template was removed as 
well.

The dub package can be found here 
https://code.dlang.org/packages/std2_format
https://github.com/burner/std2.format

When compiling the below format call with ldc and -ftime-trace
```
import std2.format;
//import std.format;

void main(){
     string s = format("Hello %s %s %.2f", "World", 1337, 13.37);
     assert(s == "Hello World 1337 13.37", s);
}

```

The overall compile time decreases from 290ms to 223ms and the 
frontend time for
the format call goes from 71ms to 23ms.

Currently, alias this and toString tests fail. And I can't really 
figure out why.
Also some float tests fails.

PR's are always welcome.

Meta: Removing wstring and dstring support from std.format for 
phobos 3 should be looked at IMHO.