Add try_insert_no_grow method on RawTable

[cole-miller]

This is my attempt to answer the need described in #224 by adding a method on RawTable:

impl<T> RawTable<T, Global> {
    pub fn try_insert_no_grow(&mut self, hash: u64, value: T) -> Result<Bucket<T>, T>;
}

This method follows the logic of RawTable::insert, but if the table would need to grow to accomodate the new element it "signals" by returning Err(value) instead of performing the reallocation and insertion.

Marked as draft for now because

• I'm not sure whether there's a better way to achieve this (advice welcome)
• some tests are probably needed
• there's an unused warning (despite the pub) that I haven't figured out how to get rid of

[cole-miller]

Fixed the unused warning by adding #[cfg(feature = "raw")] -- not sure if that's the best solution.

The latest CI failure seems to be due to a (new?) Clippy lint ptr_as_ptr triggering on other parts of the code.

[Amanieu]

Fixed the unused warning by adding #[cfg(feature = "raw")] -- not sure if that's the best solution.

That's the correct solution.

Code LGTM, just need to wait for #230 to land first.

[Amanieu]

@bors r+

[bors]

📌 Commit 92677e0 has been approved by Amanieu

[bors]

⌛ Testing commit 92677e0 with merge 4c3ba70...

[bors]

☀️ Test successful - checks-travis
Approved by: Amanieu
Pushing 4c3ba70 to master...

[cole-miller]

Thanks!