Adding a simple `coll?` function to the core

* hy/core/language.hy:
  -Added a simple coll? function that checks whether the given argument
  is an iterable and not a string,
  - Also replaced the check in `flatten` by coll?

* tests/native_tests/core.hy: Tests updated for checking coll?
docs: update docs with coll? functionality
Merge branch 'f/is-coll' of https://github.com/theanalyst/hy into theanalyst-f/is-coll
Fix indentation of *exports*