Add Checkstyle rule to enforce use of class literals for primitives & void

This commit introduces Checkstyle rules for `main` and `test` which
enforce the use of class literals for primitive types and void by
forbidding the use of the TYPE constants in Boolean, Character, Byte,
Short, Integer, Long, Float, Double, and Void.

For example, if MyClass uses one of the TYPE constants, the build will
now fail with a message similar to the following.

[ERROR] <...>/MyClass.java:39: Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE.

Author: sbrannen

gradle/config/checkstyle/checkstyleMain.xml

@@ -20,6 +20,13 @@
 		<module name="IllegalImport">
 			<property name="illegalPkgs" value="org.jetbrains.annotations" />
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="primitiveClassLiterals"/>
+			<property name="maximum" value="0"/>
+			<property name="format" value="(Boolean|Character|Byte|Short|Integer|Long|Float|Double|Void)\.TYPE"/>
+			<property name="message" value="Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE."/>
+			<property name="ignoreComments" value="true"/>
+		</module>
 	</module>
 
 	<module name="JavadocPackage" />

gradle/config/checkstyle/checkstyleTest.xml

@@ -12,6 +12,13 @@
 		<module name="IllegalImport">
 			<property name="illegalPkgs" value="org.jetbrains.annotations" />
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="primitiveClassLiterals"/>
+			<property name="maximum" value="0"/>
+			<property name="format" value="(Boolean|Character|Byte|Short|Integer|Long|Float|Double|Void)\.TYPE"/>
+			<property name="message" value="Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE."/>
+			<property name="ignoreComments" value="true"/>
+		</module>
 	</module>
 
 	<module name="SuppressWarningsFilter" />