Add heapless to Common > General > Stack-allocated arrays (and other data structures)

[ariasanovsky]

Alongside arrayvec and tinyvec, heapless has stack-allocated arrays, but also includes:

• Arc – like std::sync::Arc but backed by a lock-free memory pool rather than #[global_allocator]
• Box – like std::boxed::Box but backed by a lock-free memory pool rather than #[global_allocator]
• BinaryHeap – priority queue
• IndexMap – hash table
• IndexSet – hash set
• LinearMap
• Object – objects managed by an object pool
• String
• Vec
• mpmc::Q* – multiple producer multiple consumer lock-free queue
• spsc::Queue – single producer single consumer lock-free queue

[djc]

With 1.6M recent downloads, I think it would make sense to add this.