fix: Prevent panic in try_reserve/try_with_capacity on capacity overflow

[AriajSarkar]

Summary

Fixes #785

Prevents panic in try_reserve() and try_with_capacity() when capacity calculations overflow. This fixes the API contract violation where try_* methods were panicking instead of returning Result.

Problem

The internal helper function to_raw_capacity() panics on arithmetic overflow:

fn to_raw_capacity(n: usize) -> usize {
    match n.checked_add(n / 3) {
        Some(n) => n,
        None => panic!("requested capacity {} too large...", n),
    }
}

[seanmonstar]

Thanks!