Add support for Java 16 records

[htsaustin]

Java 16 has added support for records under JEP 395, of the form.

public record Point(int x, int y) {
}

It would be great to support these in JavaPoet, as among other things, they significantly reduce boilerplate.

[rbair23]

I am taking a crack at a PR that would add record support. However, after forking and doing a fresh build/test, I'm seeing two tests fail in IntelliJ (when running through the IDE, not in Maven).

ClassNameTest::classNameFromTypeElementDoesntUseGetKind fails:

java.lang.NullPointerException: Cannot invoke "com.sun.tools.javac.code.Symbol$Completer.complete(com.sun.tools.javac.code.Symbol)" because "c" is null

	at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:682)
	at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1410)
	at jdk.compiler/com.sun.tools.javac.code.Symbol.apiComplete(Symbol.java:688)
	at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.getKind(Symbol.java:1476)
	at com.squareup.javapoet.ClassNameTest.preventGetKind(ClassNameTest.java:135)
	at com.squareup.javapoet.ClassNameTest.classNameFromTypeElementDoesntUseGetKind(ClassNameTest.java:122)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)

and ParameterSpecTest::parameterVariableElement fails:

expected: java.lang.String arg0
but was : java.lang.String bar
Expected :java.lang.String arg0
Actual   :java.lang.String bar
<Click to see difference>


	at com.squareup.javapoet.ParameterSpecTest.parameterVariableElement(ParameterSpecTest.java:121)

Is this expected?

[vkhodygo]

@rbair23 It looks like the latter can be fixed by replacing

.isEqualTo("java.lang.String arg0");

with

.isEqualTo("java.lang.String bar");

It's not clear to me yet what causes the other one.