Minor improvements to Ion Schema Model

[popematt]

Issue #, if available:

None

Description of changes:

• Updates the AnnotationsV2 model to have special support for the simplified syntax, since it was discovered that there are significant performance improvements by doing so. (See Improves performance of annotations constraint #287) Also updates the schema reader and SchemaSymbolUtil accordingly.
• Fixes a typo in ContinuousRange.toString()
• Adds factory functions that will allow users to create a Fields constraint directly from TypeArguments instead of requiring VariableOccurringTypeArguments.

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 is 66.66% of modified lines.

Files Changed	Coverage
...reader/internal/constraints/AnnotationsV2Reader.kt	33.33%
...in/kotlin/com/amazon/ionschema/model/Constraint.kt	66.66%
...tlin/com/amazon/ionschema/model/ContinuousRange.kt	100.00%
...lin/com/amazon/ionschema/util/SchemaSymbolsUtil.kt	100.00%

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