Add new macro REF, a variadic and more general XREF

[JakobOvrum]

Usage is like:

$(REF File.writeln, std, stdio)
$(REF splitter, std, algorithm, iteration)
$(REF_ALTTEXT forward range, isForwardRange, std, range, primitives)

Added to dlang.org.ddoc because that's where XREF is defined. Can be used to link to symbols rooted in std, core, etc or any other root package.

Implementation is a bit messy. I don't know quite why, but using TAIL instead of those helper macros didn't work. Commas showed up in the output.

Will implement for DDox output ASAP.

[JakobOvrum]

Pushed a DDox implementation but I have no clue how to test it.

[JakobOvrum]

Pushed dlang/phobos/pull/3911 as an example of how it replaces existing macros.

[CyberShadow]

I don't know quite why, but using TAIL instead of those helper macros didn't work. Commas showed up in the output.

IIRC the situation is something along the lines of - some of the parameter macros (don't remember which exactly, one of $0 / $+ / etc.) pass their content as a series of parameters (which get expanded to $1/$2/...), but some pass it as a single string. This is what allows $(COMMA) to work inside nested macros AFAIU.

[andralex]

See @adamdruppe 's recent PR for simple macros that prefix with dots, underscores etc.

[JakobOvrum]

Moved the new *_PREFIXED macros from std.ddoc to macros.ddoc and used them in REF (which is in dlang.ddoc).

[JakobOvrum]

OK, can confirm that the DDox override works.

[JakobOvrum]

Removed duplicate DOT_PREFIXED_SKIP definition from std-ddox.ddoc.

[JakobOvrum]

Alright, now REF and MREF and all helper macros are all in dlang.org.ddoc. Also changed MREF to use $(D …) for the link text.

edit: Actually, most existing module links in Phobos do not use $(D …), so I reverted that.

[JakobOvrum]

REF and MREF now use ROOT_DIR in their paths.

[CyberShadow]

LGTM :)

[JakobOvrum]

Thanks!