remove dmacro.d dependency on doc.d

[WalterBright]

Also make declarations private by default and obsolete C++ linkage, and default to @trusted.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#15470"

implemented change

[dkorpel]

Instead of duplicating this function, a common function should be imported from dmd.root.utf. Otherwise, this function needs a unittest and correct documentation. ("length of an UTF string" is wrong, it returns the number of UTF-8 code units in a code point based on the first one)

[WalterBright]

I fixed the documentation and added some unittests. Generally, you're right that one shouldn't repeat oneself. In this case, I wanted dmacros.d to be usable as a macro text processor on its own, and utfStride() is a trivial function. root.utf is a substantial module. In this case, it's justifiable for a tiny bit of redundancy.

[WalterBright]

@dkorpel ping