Write support for Ion 1.1 system symbols

[popematt]

Issue #, if available:

None

Description of changes:

• Updates the FlexSym related methods in WriteBuffer to be able to write system symbols.
• Adds APIs that accept SystemSymbol to IonRawWriter_1_1.
• Adds support for writing Ion 1.1 system symbols for both the text and binary writers.
• Updates IonManagedWriter_1_1 to use system symbols for writing encoding directives and to move all of the system symbols out of user-symbol space.
• (Temporarily) disabled tests that use the Ion 1.1 reader and writer for a round trip of any sort because the reader does not have full support for system symbols yet.

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

[zslayton]

Nit: reading this I was a bit thrown off by writeAnnotations being plural, annotation0 being singular, and SystemSymbols_1_1 being plural. Could this flavor of the method have a singular name?

writer.writeAnnotation(SystemSymbols_1_1.ION_ENCODING)

I see it's part of an interface--if changing it comes with a lot of baggage then just leave it.

[popematt]

I intentionally made it plural because it overloads the existing writeAnnotations methods which are intended to provide a pseudo-varags API without the overhead of varargs.

    fun writeAnnotations(annotation0: Int)
    fun writeAnnotations(annotation0: Int, annotation1: Int)
    fun writeAnnotations(annotations: IntArray)
    fun writeAnnotations(annotation0: CharSequence)
    fun writeAnnotations(annotation0: CharSequence, annotation1: CharSequence)
    fun writeAnnotations(annotations: Array<CharSequence>)

I'm fine with changing it, but I'd prefer deal with the naming later—perhaps as part of a larger-scale review/revisiting of new API names/signatures before merging the Ion 1.1 branch into the main branch.