std.variant: Fix invalid unittest

http://d.puremagic.com/issues/show_bug.cgi?id=10879
Disable strict aliasing for now

http://d.puremagic.com/issues/show_bug.cgi?id=10750
http://forum.dlang.org/thread/l53m4a$2mum$1@digitalmars.com
Relax required precision in unit tests
Fix std.math.NaN for systems with 64bit reals
Make test value exact
Implement parse!real for systems with 64bit reals

and unittest fixes.