Shrink derived code size with with_recursive_count.

[nnethercote]

The amount of code generated by derive(Arbitrary) is large, mostly due to the recursion count guard. This commit factors out that code with a new hidden function with_recursive_count.

Because with_recursive_count is mark with inline, the generated code should end up being much the same. But compile times are reduced by 30-40% because rustc's frontend has much less code to chew through.

[nnethercote]

An example: for struct S(u32, u32);, the old derived arbitrary function looked like this:

        fn arbitrary(
            u: &mut arbitrary::Unstructured<'arbitrary>,
        ) -> arbitrary::Result<Self> {
            let guard_against_recursion = u.is_empty();
            if guard_against_recursion {
                RECURSIVE_COUNT_S
                    .with(|count| {
                        if count.get() > 0 {
                            return Err(arbitrary::Error::NotEnoughData);
                        }
                        count.set(count.get() + 1);
                        Ok(())
                    })?;
            }
            let result = (|| {
                Ok(
                    S(
                        arbitrary::Arbitrary::arbitrary(u)?,
                        arbitrary::Arbitrary::arbitrary(u)?,
                    ),
                )
            })();
            if guard_against_recursion {
                RECURSIVE_COUNT_S
                    .with(|count| {
                        count.set(count.get() - 1);
                    });
            }
            result
        }

it now looks like this:

        fn arbitrary(
            u: &mut arbitrary::Unstructured<'arbitrary>,
        ) -> arbitrary::Result<Self> {
            arbitrary::details::with_recursive_count(
                u,
                &RECURSIVE_COUNT_S,
                |mut u| {
                    Ok(
                        S(
                            arbitrary::Arbitrary::arbitrary(u)?,
                            arbitrary::Arbitrary::arbitrary(u)?,
                        ),
                    )
                },
            )
        }

The story is very similar for arbitrary_take_rest.

[fitzgen]

Nice, thanks!