Calling `splitter` with both a predicate and a string for the separator matches two overloads

[jmdavis]

This code fails to compile

void main()
{
    import std.algorithm.iteration : splitter;
    import std.algorithm.searching : canFind;
    import std.typecons : Yes;

    auto range = "16x16+0-2".splitter!((a, b) => "x+-".canFind(a), Yes.keepSeparators)("x");
}

giving

q.d(7): Error: `std.algorithm.iteration.splitter` called with argument types `(string, string)` matches multiple overloads exactly:
/usr/local/include/dmd/std/algorithm/iteration.d(5338):     `std.algorithm.iteration.splitter!((a, b) => "x+-".canFind(a), Flag.yes, string, string).splitter(string r, string s)`
and:
/usr/local/include/dmd/std/algorithm/iteration.d(5909):     `std.algorithm.iteration.splitter!((a, b) => "x+-".canFind(a), Flag.yes, string, string).splitter(string r, string s)`
auto range = "16x16+0-2".splitter!((a, b) => "x+-".canFind(a), Yes.keepSeparators)("x");
^

This is contrast to

void main()
{
    import std.algorithm.iteration : splitter;
    import std.algorithm.searching : canFind;
    import std.typecons : Yes;

    auto range = "16x16+0-2".splitter!((a, b) => "x+-".canFind(a), Yes.keepSeparators)('x');
}

which compiles just fine (though is buggy with front as shown in #10759).

So, the template constraints need work.