modularize `assert_failed` panic calls

[alejandro-vaz]

some SmallVec methods use assertions that rely on a helper function marked as #[cold] and never inlined. this is to improve cache locality

take all the helper functions like that, rename them, and put it in an assertions.rs module

example in SmallVec::remove:

    #[inline]
    pub fn remove(&mut self, index: usize) -> T {
        #[cold]
        #[inline(never)]
        #[track_caller]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("removal index (is {index}) should be < len (is {len})");
        }

        let len = self.len();
        if index >= len {
            assert_failed(index, len);
        }
        let new_len = len - 1;
        unsafe {
            // SAFETY: new_len < len
            self.set_len(new_len);
            let ptr = self.as_mut_ptr();
            let ith = ptr.add(index);
            // This item is initialized since index < len
            let ith_item = ith.read();
            copy(ith.add(1), ith, new_len - index);
            ith_item
        }
    }