Attribute macro that expands function definition to `RecurFn` implementation.

[Jason5Lee]

I think it's a good idea to have a attribute macro, which, for example, expands

#[recur_fn]
fn fact(n: u64) -> u64 {
    if n == 0 { 1 } else { n * fact(n - 1) }
};

to

struct Fact {}
impl RecurFn<u64, u64> for Fact {
    #[inline]
    fn body(&self, fact: impl Fn(u64) -> u64, n: u64) -> u64 {
        if n == 0 { 1 } else { n * fact(n - 1) }
    }
}
static FACT: Fact = Fact {};
fn fact(n: u64) -> u64 {
    FACT.call(n)
}