static ARENA_ID : Mutex Global identificator generator for all 'struct Arena<T>' instances. TODO: Need to wrap to Arc for thread synchronization. Rename to ARENA_ID.
pub struct Arena<T> 'id' is just 'ARENA_ID + = 1' in a 'Arena::new' method. The "heap" field contains the vaalue 'None' placed in the 'freed' field, the value 'Some' is the valid instance of 'Some(T)'. The "freed" field contains the freed heap indexes.