ruleguard: add support for local functions

[quasilyte]

This feature is useful for rule filters readability improvements.

Instead of copying a complex Where() expression several times,
one can now use a local function literal to define that filter
operation and use it inside Where() expressions.

Here is an example:

func preferFprint(m dsl.Matcher) {
	isFmtPackage := func(v dsl.Var) bool {
		return v.Text == "fmt" && v.Object.Is(`PkgName`)
	}

	m.Match(`$w.Write([]byte($fmt.Sprint($*args)))`).
		Where(m["w"].Type.Implements("io.Writer") && isFmtPackage(m["fmt"])).
		Suggest("fmt.Fprint($w, $args)").
		Report(`fmt.Fprint($w, $args) should be preferred to the $$`)

	m.Match(`$w.Write([]byte($fmt.Sprintf($*args)))`).
		Where(m["w"].Type.Implements("io.Writer") && isFmtPackage(m["fmt"])).
		Suggest("fmt.Fprintf($w, $args)").
		Report(`fmt.Fprintf($w, $args) should be preferred to the $$`)

	// ...etc
}

Note that we used isFmtPackage in more than 1 rule.

Functions can accept almost arbitrary params, but there are some
restrictions on what kinds of arguments they can receive right now.

These arguments work:

• Matcher var expressions like m["varname"]
• Basic literals like "foo", 104, 5.2
• Constants