add byCodeunit, byChar, byWchar and byDchar to std.utf

[Walter Bright]

We've talked about adding these algorithms at length here in the n.g., so here 
they are.

https://github.com/D-Programming-Language/phobos/pull/2043

They're intended as 'adapter' style algorithms.

The .byCodeunit is intended as "by gum, I don't want auto-decode". byCodeunit 
was suggested earlier in the n.g., I don't recall who (speak up!).