fix std/range/constraints.d static errors

remove scope import in std.range.constraints.

use string concatenation

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fix Issue 13746 - formatValue of delegates
Merge pull request #2736 from 9il/erc

fix std/range/constraints.d static errors
Document new idiom for using Flag.
Merge pull request #2745 from 9il/fv

fix Issue 13746 - formatValue of delegates
Merge pull request #2741 from quickfur/flag

Document new idiom for using Flag.
std.conv.d - Fix typo
Merge pull request #2748 from JinShil/patch-1

std.conv.d - Fix typo