Relax constraint requiring that AutoFilters have unique output types

[codemercenary]

Should be able to declare an AutoFilter method that has the same argument type twice as an output without problems, as this behavior is already supported for lambda functions and behaves the way one might expect.

[jnguyen75]

Neat trick to count the args in test:
*factory += [&] (const Decoration<0>* args []) { while (*args++) nArgs++; };

Builds and tests pass on mac. Can merge when Travis passes

[codemercenary]

Yep, I like the dereference-increment-test idiom. When I'm iterating a string, I use the same trick--finding the length of a string, for instance:

for(size_t i = 0; *p++; i++);