A<B<C...; slow

[guwirth]

Parsing of the below rare use case is still slow. (leftover #1685)

still slow:

void f() {
   ss < a < b < c < d < e < f < g;
}

fast:

using List= TypeA<A, TypeB<B, TypeC<C, TypeD<D, TypeE<E, TypeF<F, TypeG<G, TypeH<H, I>>>>>>>>;

void f1() {
   ss << a << b << c << d << e << f << g;
}

void f2() {
   TypeA<A, TypeB<B, TypeC<C, TypeD<D>>>>;
}

void f3() {
   TypeA<A, TypeB<B, TypeC<C, TypeD<D;
}

[guwirth]

From #2206 (slow):

#include <iostream>
#include <string>
#include <sstream>

std::ostream& operator< (std::ostream& dest, const std::string& what) { 
   return dest;
}

int main () 
{
   std::string a, b, c, d, e, f;
   std::stringstream dest;
   
   dest < a < b < c < d < e < f;
}