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`.


What's the purpose of this change?