Should we introduce keys of any type in map/record/dictionary in AskScript?

Following [this comment](https://github.com/xFAANG/askql/pull/259#discussion_r446513938), let's discuss whether we should allow arbitrary type of keys in AskScript associative types and whether we should extend the current type (map) or rather introduce a new type (record, dict or whatever other name).

Start with the linked discussion in pull request #259, then review the existing AskScript associative-type design. Decide whether arbitrary key types belong in the current map or require a new record or dictionary type; done means the choice and intended type behavior are clearly specified. No implementation files or tests are named in the issue.

Written by the indexing model from the issue text.