Move variable declarations that are top level in function scope, to the function parameters. #1209

Here is a real world example, that has been formatted for readability purposes, of what terser gives me in one of my libraries:

export var n = (e) => {
		throw Error(e);
	},
	m = new WeakMap(),
	F = (e) => {
		let t = m.get(e);
		return t || n(0), t;
	};

This can be further minified like this:

export var n = (e) => {
		throw Error(e);
	},
	m = new WeakMap(),
	F = (e,t = m.get(e)) => t || n(0), t;

let and return and {,} are stripped. This is 14 characters in total, and given that we need to add a new , in the function parameters, we end up using 13 characters less.

This should be done only for functions that belong to the private api. (The example I provided actually belongs to the private api of my library. The export is used just for terser to not dead code eliminate the whole code)

Edit : Care should be taken for this to not be applied to function that gather the arguments with the triple dot operator.