Adds changes to read full schema document

[popematt]

Issue #, if available:

#256

Description of changes:

• Changes the signature of readSchema so that it returns a Pair<SchemaDocument, Iterator<IonValue>>—essentially, it returns the SchemaDocument and any unread IonValues.
• Adds some utility functions for identifying top level content (header, footer, type, open content).
• Adds implementation of readSchema with a lot of tests.
• Fixes a bug with the reportError function that showed up while testing readSchema. Basically, fast-fail wasn't working properly because it was only throwing on fast fail without adding the error to the ReadContext.
• Another incidental change is adding a useful toString() for Bag. I realized that the default impl was giving me things like Bag@398c0a, so I changed it to something useful.

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:

None.

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

[codecov]

Codecov Report

Patch coverage: 84.50% and project coverage change: +0.67 🎉

Comparison is base (63aba66) 83.59% compared to head (b5bbfa3) 84.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   83.59%   84.27%   +0.67%     
==========================================
  Files         133      133              
  Lines        3262     3325      +63     
  Branches      768      788      +20     
==========================================
+ Hits         2727     2802      +75     
+ Misses        306      295      -11     
+ Partials      229      228       -1     

Impacted Files	Coverage Δ
...azon/ionschema/internal/util/IonValueExtensions.kt	80.00% <0.00%> (-8.89%)	⬇️
...a/src/main/kotlin/com/amazon/ionschema/util/Bag.kt	75.00% <0.00%> (-10.72%)	⬇️
...otlin/com/amazon/ionschema/reader/internal/util.kt	81.81% <81.81%> (+18.18%)	⬆️
...com/amazon/ionschema/reader/IonSchemaReaderV2_0.kt	88.33% <87.03%> (+34.48%)	⬆️
...lin/com/amazon/ionschema/reader/IonSchemaReader.kt	100.00% <100.00%> (+66.66%)	⬆️
.../amazon/ionschema/reader/internal/ReaderContext.kt	92.30% <100.00%> (+0.64%)	⬆️
...amazon/ionschema/reader/internal/TypeReaderV2_0.kt	93.24% <100.00%> (+2.96%)	⬆️

... and 3 files with indirect coverage changes

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