Add iterable concept

[tcbrindle]

[codecov[bot]]

Codecov Report

:x: Patch coverage is 98.14634% with 19 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 96.85%. Comparing base (e8936e5) to head (c4a5513).

Files with missing lines	Patch %	Lines
include/flux/adaptor/chunk.hpp	92.85%	3 Missing :warning:
include/flux/adaptor/take_while.hpp	78.57%	3 Missing :warning:
test/test_utils.hpp	87.50%	1 Missing and 2 partials :warning:
include/flux/adaptor/chain.hpp	94.11%	2 Missing :warning:
include/flux/factory/generator.hpp	92.00%	2 Missing :warning:
include/flux/adaptor/flatten_with.hpp	97.36%	1 Missing :warning:
include/flux/adaptor/scan_first.hpp	94.73%	1 Missing :warning:
include/flux/algorithm/to.hpp	94.11%	1 Missing :warning:
include/flux/algorithm/zip_algorithms.hpp	94.44%	1 Missing :warning:
test/test_generator.cpp	75.00%	0 Missing and 1 partial :warning:
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
- Coverage   97.74%   96.85%   -0.90%     
==========================================
  Files         146      148       +2     
  Lines        5149     5147       -2     
  Branches      244      301      +57     
==========================================
- Hits         5033     4985      -48     
- Misses         80      127      +47     
+ Partials       36       35       -1     

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