Crate common_macros

This library will contains common macros.

Mainly following macros:

• hash_map!,
• hash_set!,
• b_tree_map!,
• b_tree_set!,
• expr_count!
• insert! (generalization of map/set/vec insertion)

I'm not sure if I will add common alt. assertion macros (e.g. assert_ok!, assert_err!, assert_iter_eq!).