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.
@DmitryOlshansky One of the template constraints is `isSomeFunction!fun`. Will that let callables other than functions and function pointers through?