Fix Issue 3632 - modify float is float to do a bitwise compare

[yebblies]

Currently floating point types are the only types where 'is' is not a bitwise comparison. This patch makes floating is floating perform a bitwise comparison, at runtime and compile-time.

http://d.puremagic.com/issues/show_bug.cgi?id=3632

[denis-sh]

What is the state of this ("hopefully last") pull which fails tests now?

[yebblies]

Doesn't work.

[yebblies]

This now works, except that this code T46!(double.nan) + T46!(-double.nan); generates two different template instances with the same mangled name. (which the win64 linker can't handle)

Not sure what to do here. Mangle each NaN differently? Make all NaNs the same at compile time? Thanks to real.init, we have a way to create at least two different NaNs at compile time.

[andralex]

I think each NaN should mangle differently. Applications are known to place useful data in NaNs' payload, which is well defined.

[ibuclaw]

Rebased in #7568