std.range.tee should accept function symbols

https://issues.dlang.org/show_bug.cgi?id=13483

When `tee` is used with function symbols, like `[1, 2, 3, 4, 5].tee!writeln`, `tee` attempts to call the function instead of passing as a pointer.
You are right it won't but then I really don't understand why limit to only true functions and delegates in the first place.