Fix fromdicts generator support lazy

[arturponinski]

This PR has the objective of improving the support of generators in fromdicts. The current implementation uses itertools.tee which according to docs and production deployments uses large amounts of memory, leading to out of memory kills of processes. This PR aims to keep the improved support of generators by using a filecache, similar to sorting, to allow multiple iterations.

Closes https://github.com/petl-developers/petl/issues/618

Changes Moved _iterchunk from sorts to petl.util.base. Imported to sorts as _iterchunk for BC

Changes

1. Refactored DictsGeneratorView in petl.io.json to use file cache in a lazy manner, dumping data during the first requested pass

Checklist

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• [ ] Source Code rules apply:
  • [x] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [x] New functions are included in API docs
  • [x] Docstrings include notes for any changes to API or behaviour
  • [x] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [x] Using atomic commits when possible
  • [x] Commits are reversible when possible
  • [x] There is no incomplete changes in the pull request
  • [x] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [x] Tested locally using tox / pytest
  • [x] Automated testing passes (see CI)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [x] Ready to review
  • [x] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 2880829446

• 95 of 97 (97.94%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 91.093%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
petl/io/json.py	46	48	95.83%
<!--	Total:	95	97	97.94%	-->

Totals
Change from base Build 2692922856:	0.04%
Covered Lines:	12713
Relevant Lines:	13956

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💛 - Coveralls