Use FixedArray instead of Box<[T]>

[GnomedDev]

This reduces the inline size of DashMap from ptr (usize) + length (usize) + shift (usize) to ptr (usize) + length (u32) + shift (u16) while removing padding which made no measurable difference in my testing. This should be benchmarked and especially checked on an ARM server to see if the padding makes a difference there.