Don't import std.format in any public scope.

Workaround for: https://issues.dlang.org/show_bug.cgi?id=13808
Add missing imports
Merge pull request #2783 from 9rnsr/fix_imports

Add missing imports
Merge pull request #2780 from quickfur/issue13808

Issue 13808: Don't import std.format in any public scope.