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`.


Hmm you're right. I must have misread it as `indent()`. :-( That will teach me to review PR's late at night when i'm too sleepy to think straight... :-P