std.uni, std.ascii, std.encoding, std.utf ugh!

[learner]

Good morning,

Trying to do this:

```
bool foo(string s) nothrow { return s.all!isDigit; }
```

I realised that the conversion from char to dchar could throw.

I need to validate and operate over ascii strings and utf8 
strings, possibly in separate functions, what's the best way to 
transition between:

```
immutable(ubyte)[] -> validate utf8 -> string -> nothrow usage -> 
isDigit etc
immutable(ubyte)[] -> validate ascii -> AsciiString? -> nothrow 
usage -> isDigit etc
string             -> validate ascii -> AsciiString? -> nothrow 
usage -> isDigit etc
```

Thank you