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`.


I think this is a bit extreme. This is a *unittest*, not a function everyone is going to use. Why does an import inside a *unittest* matter so much? If it's actually a problem, then we need to be addressing the root cause of the problem (on the forum) instead of just randomly removing things from Phobos unittests.