Fix Phobos dependencies on autodecoding

[Vladimir Panteleev]

On Thursday, 15 August 2019 at 12:09:02 UTC, Vladimir Panteleev 
wrote:
> I haven't looked into that, perhaps someone more familiar with 
> std.regex and std.uni could have a look.

In std.uni, there is genericDecodeGrapheme, which needs to:

1. Work with strings of any width
2. Work with input ranges of dchars
3. Advance the given range by ref

With autodecoding on, the first case is handled by .front / 
.popFront.

With autodecoding off, there is no direct equivalent any more. 
The problem is that the function needs to peek ahead (which can 
be multiple range elements for ranges of narrow char types, which 
is not possible for input ranges).

- Replacing .front / .popFront with std.utf.decodeFront does not 
work because the function does not do these operations in the 
same place, so we need to save the range before decodeFront 
advances it, but we can't .save() input ranges from case 2 above.

- Using byDchar does not work because .byDchar does not take its 
range by ref, so advancing the byDchar range will not advance the 
range passed by ref to genericDecodeGrapheme. I tried to use 
std.range.refRange for this but hit a compiler ICE ("precedence 
not defined for token 'cantexp'").

Perhaps there is already a construct in Phobos that can solve 
this?