Speed up the build (#169)

The build is sped up in two ways:
1. By tweaking the JVM flags passed to the main process.
2. By tweaking the JVM flags passed to the forked Surefire processes.

.mvn/jvm.config

@@ -1,3 +1,6 @@
+-XX:ReservedCodeCacheSize=512m
+-XX:SoftRefLRUPolicyMSPerMB=10
+-XX:+UseParallelGC
 --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
 --add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
 --add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED

pom.xml

@@ -75,11 +75,14 @@
         arguments. In particular, JaCoCo relies on this for the configuration
         of its Java agent. -->
         <argLine>
-            <!-- Error Prone makes extensive use of unsupported
+            <!-- The JVM arguments specified in `.mvn/jvm.config` also apply to
+            test JVMs, as those are also non-interactive processess running
+            Error Prone, and thus similarly make extensive use of unsupported
             `javac`-internal APIs, in some cases even reflectively inspecting
-            them. These APIs are accessed at compile time and when running
-            tests. The former case is covered by flags in `.mvn/jvm.config`;
-            the latter is covered here. -->
+            them. -->
+            -XX:ReservedCodeCacheSize=512m
+            -XX:SoftRefLRUPolicyMSPerMB=10
+            -XX:+UseParallelGC
             --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
             --add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
             --add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED
@@ -93,9 +96,10 @@
             <!-- The test JVMs are short-running. By disabling certain
             expensive JIT optimizations we actually speed up most tests. -->
             -XX:TieredStopAtLevel=1
-            <!-- We cap memory usage. This is primarily relevant for build agents,
-            but locally this should also be more than enough. -->
-            -Xmx512m
+            <!-- We cap memory usage. This may be relevant for build agents,
+            but also prevents excessive memory usage by heavily parallelized
+            local builds. -->
+            -Xmx1024m
             <!-- This argument cannot be set through Surefire's
             'systemPropertyVariables' configuration setting. Setting the file
             encoding is necessary because forked unit test invocations