Add support for void[0] types.

[ghost]

The void[0][Key] trick is used when you want to use builtin hashes as Set types (hashes with no values).

[repeatedly]

This is interesting pull request :)
void[0] hash value type as set is popular?

[ghost]

I've seen it mentioned in several NG posts that using such a value type means the hashmap won't use any memory for the values, but I never checked this.

[ghost]

I think it could be true because:

alias void[0] Type;
writeln(Type.sizeof);  // writes 0

[ghost]

Ok I've significantly simplified this now, instead of hardcoding hash support everywhere I've simply implemented pack, unpack and as for void[0] type. So now this will automatically work for hashes having that type as a value.

[ghost]

Ah the commits somehow reference an older one, let me fix this first.