Consider concepts from C++ unordered_map library

[wolfram77]

Take into account these improvements. Additionally, provide any references to functions that were left out.

Capacity

• empty
• size

Modifiers

• clear
• insert
• insert_or_assign
• erase
• swap
• extract
• merge

Lookup

• count
• find
• contains
• equal_range

Bucket interface

• bucket_count
• max_bucket_count
• bucket_size
• bucket

Hash policy

• load_factor
• max_load_factor
• rehash
• reserve

Observers

• hash_function
• key_eq