std.stdio: clean imports

fix

fix

remove string import

uncomment unittest

update Flag!"KeepTerminator" usage

fix typo

fix errors

private format

fix yes/no

update private format

format workaround

renamed flags to lower case
Fix missing local import in std.math.expi.
use lower-case flag strings

- so that Yes.keepTerminator/No.keepTerminator don't look so weird
Merge pull request #2727 from 9il/stdio

std.stdio: clean imports
Merge pull request #2743 from MartinNowak/fixup2731

use lower-case flag strings
Merge pull request #2742 from quickfur/expi_import

Fix missing local import in std.math.expi.