Feature: naive support for const param generics.

[Y-jiji]

My Case:
When I was working on creating a scanner for log structured merge tree, I have to implement a Scanner trait that supports scanning keys inside a key range.
It features:

• either a forward or reverse scan.
• keep the scanned block on disk (which means the underlying iterator self-references an Arc)
  Therefore, I have to write a type like:

pub trait Scannable {
    type K; type V;
    type Scanner<'a, const RVS: bool>:
        for<'b> IteratorExt<Item<'b> = (&'b Self::K, &'b Self::V)>
        where Self: 'a;
    fn scan<const RVS: bool>(&self, range: impl RangeBounds<Self::K>) -> Self::Scanner<'_, RVS>;
}

/// with reference
#[ouroboros::self_referencing]
pub struct SelfRefScanner<T, const RVS: bool>
where
    T: Scannable + 'static,
{
    orig: Arc<T>,
    #[borrows(orig)]
    #[not_covariant]
    proj: T::Scanner<'this, RVS>,
}

Changed Behaviour:
In current implementation the macro panics without giving any reason, as suggested in #47 .
The tweaked implementation supports constant parameters.

Soundness:
Since the added constants don't carry additional data from runtime, it doesn't effect validity.
Since the number of added constants are finite, an exhausting the finite space proofs type safety, as long as the compiler doesn't raise any errors.

[Y-jiji]

@joshua-maros I need this badly. Please merge ASAP if there exists no further concerns.

[someguynamedjosh]

LGTM, I've published this as version 0.18.1.