Expose owned sequence iterator; Add Owned Field iterator.

[jpschorr]

Follow-up to #858

This PR includes a couple changes intended to make working with owned Elements easier and not require cloning for 'projecting' an element to be replaced by something it contains.

• Exposes ConversionOperationError and ConversionOperationResult
• Adds ability to retrieve original value from ConversionOperationError via original_value()
• Adds OwnedFieldIterator for iterating struct fields of owned Structs

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Attention: Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.84%. Comparing base (f3b6cca) to head (dfcc720).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/result/conversion.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #866      +/-   ##
==========================================
+ Coverage   77.83%   77.84%   +0.01%     
==========================================
  Files         136      136              
  Lines       34205    34235      +30     
  Branches    34205    34235      +30     
==========================================
+ Hits        26623    26650      +27     
- Misses       5626     5629       +3     
  Partials     1956     1956              

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