Change IonSystemBuilder's default charset to UTF-8

[popematt]

Issue #, if available:

Fix #783

Description of changes:

• I decided to change the default charset for IonSystemBuilder to UTF-8
• I looked into allowing the different reader/writer builders to use different catalogs, but it turns out that the IonSystem does interact with the catalog outside of the reader/writer builders through createSharedSymbolTable(). I don't think there's a good way to define how that method should interact with multiple catalogs since a shared symbol table could be present in one catalog but not in another.
• The changes in IonSystemLite are all non-functional. I cleaned up some formatting near the reader/wwriter builder member fields (and updated the copyright header).

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.41%. Comparing base (3c1b6b1) to head (bdf84c6).
Report is 34 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #802      +/-   ##
============================================
+ Coverage     67.23%   67.41%   +0.17%     
- Complexity     5484     5518      +34     
============================================
  Files           159      160       +1     
  Lines         23025    23077      +52     
  Branches       4126     4126              
============================================
+ Hits          15481    15557      +76     
+ Misses         6262     6246      -16     
+ Partials       1282     1274       -8     

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[tgregg]

Will approve as long as a dry run looks good