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.

I should add that the std.uni "silent" breakage also was due to 
`dchar c = str.front`, and would have been found by disallowing 
char->dchar implicit conversion.