Win compilation fail. Forward reference of nullSlotDummy and slotCompare.

Platform: Windows 7 x86_64
DMD: 2.062
Log:
```
Checking dependencies in 'D:\dev\d\foguansite'
Building configuration "application", build type debug
Copying files...
Running dmd (compile)...
.dub\packages\vibe-d\source\vibe\core\drivers\libevent2.d(336): Error: template instance vibe.utils.hashmap.HashMap!(Thread, ThreadSlot, nullSlotDummy, slotCompare) forward reference of function nullSlotDummy
.dub\packages\vibe-d\source\vibe\core\drivers\libevent2.d(336): Error: template instance vibe.utils.hashmap.HashMap!(Thread, ThreadSlot, nullSlotDummy, slotCompare) forward reference of function slotCompare
.dub\packages\vibe-d\source\vibe\core\drivers\libevent2.d(336): Error: template instance vibe.utils.hashmap.HashMap!(Thread, ThreadSlot, nullSlotDummy, slotCompare) error instantiating
C:\D\dmd2\windows\bin\..\..\src\phobos\std\range.d(611): Error: static assert  "Cannot put a char[] into a Appender!(string)"
C:\D\dmd2\windows\bin\..\..\src\phobos\std\format.d(1436):        instantiated from here: put!(Appender!(string), char[])
C:\D\dmd2\windows\bin\..\..\src\phobos\std\format.d(1338):        instantiated from here: formatUnsigned!(Appender!(string), char)
C:\D\dmd2\windows\bin\..\..\src\phobos\std\format.d(1312):        instantiated from here: formatIntegral!(Appender!(string), ulong, char)
C:\D\dmd2\windows\bin\..\..\src\phobos\std\format.d(1071):        ... (12 instantiations, -v to show) ...
C:\D\dmd2\windows\bin\..\..\src\phobos\std\uuid.d(371):        instantiated from here: parse!(ubyte, string)
C:\D\dmd2\windows\bin\..\..\src\phobos\std\uuid.d(1468):        instantiated from here: __ctor!(immutable(char))
Error: Build command failed with exit code 1

Run 'dub help' for usage information.
```

Easy fix to move nullSlotDummy and slotCompare  above the alias.