move format to std.format

Blocked by DMD Issue: https://issues.dlang.org/show_bug.cgi?id=13745
History of this PR with commits: https://github.com/9il/phobos/tree/move2
See also: #2737 

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The reason is to do not import `std.uni` tables, `std.string` and
probably `std.algorithm`.

Public import `format` added in `std.string` with docs like `icmp`.


It's a private CTFE function. I don't think it's a big deal. It's probably there to help with debugging (nicer indentation in the generated mixin string), since the compiler doesn't really care and it makes no difference in the generated code.