Clean up unexpected interactions between `IonSystemBuilder` and the reader and writer builders

[popematt]

• The default charset for TextWriterBuilder is UTF-8. However, IonSystemBuilder uses ASCII as the charset for its own default TextWriterBuilder.
• The IonSystemBuilder ignores the catalog of any provided reader builder or writer builder, and in the IonSystemLite constructor, it even asserts that all of the catalogs are the same.

See #781 for some discussion of these issues.