Ensure that all empty PersistentLists are the same instance.

[popematt]

Issue #, if available:

None

Description of changes:

There's a problem in kotlinx.collections.immutable (described in Kotlin/kotlinx.collections.immutable#176) that was causing ion-element-kotlin to unnecessarily allocate large numbers of empty lists for unannotated values (and anywhere else that had a possibly-empty list).

Unfortunately, we won't be able to use the fix from that PR until we can update ion-element-kotlin to Kotlin >= 1.9.
This adds our own extension functions to work around the bug.

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 88.37209% with 5 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (master@5b9ec91). Click here to learn what that means.

Files	Patch %	Lines
src/com/amazon/ionelement/api/Ion.kt	81.48%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #91   +/-   ##
=========================================
  Coverage          ?   90.01%           
  Complexity        ?      496           
=========================================
  Files             ?       31           
  Lines             ?     1062           
  Branches          ?      157           
=========================================
  Hits              ?      956           
  Misses            ?       63           
  Partials          ?       43           

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[jobarr-amzn]

Looks good, thanks for finding and fixing this!