Optimizes IonValue.getType() to avoid vtable/itable lookups when invoked on IonStruct values.

[tgregg]

Description of changes:
Also adds a 'jmh' target that allows us to add microbenchmarks for targeted performance testing. These benchmarks are run using ./gradlew jmh. Example output:

Benchmark                                  Mode  Cnt    Score   Error  Units
IonValueGetTypeBenchmark.getTypeContainer  avgt    5  469.625 ± 5.799  ns/op
IonValueGetTypeBenchmark.ionValueGetType   avgt    5    7.863 ± 0.139  ns/op

This is optional and can be removed from this PR if reviewers have objections. I think it's nice because it reduces the effort required to add new JMH benchmarks that are more targeted than the ion-java-benchmark-cli. Note that we followed this same pattern in ion-java-path-extraction. New benchmarks need not even be committed, but having the JMH framework committed allows for quick local experimentation. I'm proposing to commit the new IonValueGetTypeBenchmark mostly as an example.

As for the proposed change, the existing implementation of IonValue.getType() is an interface method with 14 implementations, requiring an itable lookup to invoke. This has been shown to be much slower than invoking a final method or an abstract method with two or fewer implementations. The proposed change adds a special case for IonStructLite.getType that avoids itable/vtable lookups. All other types fall back to the existing implementations, now called getTypeSlow().

IonStructLite is chosen for the fast path because it is the most commonly used container type as well as one of the most common of all types. This is important because internal usages of IonValue.getType() occur on container values much more often than on scalars. Although the added microbenchmarks show this change is performance-neutral, performance testing within a large application that heavily uses Ion structs demonstrates significant benefit.

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

[jobarr-amzn]

I see you included the most important part of the optimization 🎉 .