feature-idea: add hint when manual change could optimize

[flocsy]

Add an option to the settings that when enabled will give warnings/hints for manual edits to improve code.

Sometimes people don't want to use the optimizer that generates the code under optimized/, but would like to learn the tricks how to change their code to make it "better". I sometimes do look at the generated code, take note of a trick (which is not always easy to understand from there), and then I try to keep that in my mind when I write code.

I don't know what things could work, but I think some will be easy to implement, some will be harder. I'll give you an example of what I think of:

function x() {
var a = mX + 3;
var b = 2 * mX +2;
}

This could give a warning that the same non-local variable is referenced more than once in the same function and it could be fetched to a local variable.

I think you get the idea.