Need to do some "dirty" UTF-8 handling

[Andrej Mitrovic]

I've had a similar requirement some time ago. I've had to copy and
modify the phobos function std.utf.decode for a custom text editor
because the function throws when it finds an invalid code point. This
is way too slow for my needs. I'm actually displaying invalid code
points with special marks (just like Scintilla), so I need decoding to
work as fast as possible.

The new function simply replaces throwing exceptions with flagging a boolean.