make segtree to allow more broad function types

[yosupo06]

I tried to make segtree / lazy_segtree can handle more broad function types.

By this change,

• We continue to support to naive function (= S op(S a, S b), ...)
• We can use other function types, e.g. S op(const S& a, const S& b) (C++17) or static lambda (C++20)

string op_const(const string& a, const string& b) {
    return a + b;
}

string e() { return ""; }

segtree<string, op_const, e> seg(10);

segtree<string, [](string a, string b) { return a + b; }, []() { return ""; }> seg(10);

(TBH I'm not sure whether this change really works well because it uses advanced C++ feature & I'm not a pro of C++)