changes Struct and StructRef implementations to consider field order

[desaikd]

Issue #200

Description of changes:
This PR changes Struct and StructRef implementations to consider field order

List of changes:

• replaces usage of HashMap with LinkedHashMap
• adds dependency to hashlink in Cargo.toml

Test:
Added a unit tests in mod.rs which tests operations on struct through generic trait IonElement.

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

[codecov]

Codecov Report

Merging #429 (f96c8b3) into main (024cb2e) will increase coverage by 0.04%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
+ Coverage   83.84%   83.89%   +0.04%     
==========================================
  Files          83       83              
  Lines       15532    15541       +9     
==========================================
+ Hits        13023    13038      +15     
+ Misses       2509     2503       -6     

Impacted Files	Coverage Δ
src/value/owned.rs	74.14% <50.00%> (ø)
src/value/borrowed.rs	65.37% <66.66%> (+1.73%)	⬆️
src/value/mod.rs	88.86% <85.71%> (+0.02%)	⬆️
src/binary/non_blocking/binary_buffer.rs	97.04% <0.00%> (-0.25%)	⬇️

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[rmarrowstone]

curious how/why this particular implementation was chosen.

[zslayton]

It was the confluence of a few superficial factors:

• It was the first search result for LinkedHashMap on crates.io.
• It had over 6 million downloads.
• It had a few high-profile dependents, including SQLx and DuckDB.
• It had an MIT/Apache 2.0 dual license.

[zslayton]

Looks good. We'll need to look at simplifying the definition of Struct to only have a linked hash map (i.e. remove the no_text_fields collection) in a future PR.