import std.typecons outside unittests since it is needed

see the code I wrote here:
http://forum.dlang.org/thread/xmusisihhbmefeigvxvd@forum.dlang.org#post-sxbwtjnchkjzfqgocoxf:40forum.dlang.org

/home/me/d/dmd2/linux/bin32/../../src/phobos/std/uni.d(6301): Error: undefined identifier tuple
[snip]
test50.d(8): Error: template instance std.uni.normalize!(cast(NormalizationForm)0, dchar) error instantiating

std.typecons is needed outside of unittests too!