Functions as `recur` target (i.e. without `loop`)

[bobbicodes]

Currently, recur may only be used with loop. It took me an entire month to properly implement loop, so I got a bit burnt out. But in some ways, using a function as a recur point is actually easier, because we already have implicit tail recursion optimization unlike Clojure. Because of this fact, any use of recur without a loop I believe can be done by simply replacing the recur with the actual function! In fact, I had this working in a previous version. We have a postwalk function which I implemented specifically for this, which is currently not being used. How sad. So I know it works, at least for all the test cases I tried. All I needed to do was check if the function being defined has a loop, and if not, walk the AST and replace any occurrence of recur with the function itself (the function object, not its name, otherwise it wouldn't work with lambdas).

[bobbicodes]

The place where this was previously handled was in the function type:

function hasLoop(ast) {
    let loops = []
    postwalk(x => {
        if (x.value == _symbol("loop")) {
            loops.push(true)
            return true
        } else {
            return x
        }
        return x
    }, ast)
    if (loops.length > 0) {
        return true
    } else {
        return false
    }
}

export function _function(Eval, Env, ast, env, params) {
    var fn = function () {
        return Eval(ast, new Env(env, params, arguments))
    }
    let swapRecur = postwalk(x => {
        if (x.value == _symbol("recur")) {
            return fn
        } else {
            return x
        }
        return x
    }, ast)
    if (!hasLoop(ast)) {
        ast = swapRecur
        fn = function () {
            return Eval(ast, new Env(env, params, arguments))
        }
    }
    fn.__meta__ = null;
    fn.__ast__ = ast;
    fn.__gen_env__ = function (args) { return new Env(env, params, args); };
    fn._ismacro_ = false;
    return fn;
}

See https://github.com/bobbicodes/exercism-express/blob/8ebc9adb921858a28983cea4c0811a2cd40ed5d8/src/types.js#L208C1-L235C2

[bobbicodes]

Fixed by #22