Would be nice to have a with_capacity() constructor for Freelist

[alexpyattaev]

Just benchmarked your freelist implementation, and it is remarkably fast. Sadly, it lacks a way to preallocate the memory which tanks performance early on. Could you please add the with_capacity constructor? It should be fairly trivial as far as I understand.
You'd just need to change this line to Vec::with_capacity() and you basically get ready-made with_capacity constructor.

narui/freelist/src/lib.rs

Line 42 in 2c758be

let mut data = vec![];

Thanks!