[opt pass] string concatenations to vector pushes

[alex-s168]

Things like:

let x = "a b " + c + "d";
x = " e" + "f";

should be replaced with:

let _x = mk-vector(type(U8),0);
_x = _x.push-all("a b ");
_x = _x.push-all(c);
_x = _x.push-all("d");
_x = _x.push-all(" e");
_x = _x.push-all("f");
let x = _x.move-to(type(String));

blockers:

• stdlib collection improvements #1250
• compiler internals should get relatively stable before implementing optimizations