Add support for unnamed anonymous fn output schema

Not sure if there was a reason for not supporting this, but this extends `sm/fn` to support a schema without an explicit name.
There's already a fn called fn-schema that does something different, so this is a bit confusing.  

Can you just add the anon cases with a `testing` here instead?

https://github.com/Prismatic/schema/blob/master/test/cljx/schema/core_test.cljx#L698

Thanks!