Replace custom maps on primitive int types with fastutil implementation.

BUG=

Change-Id: I8bac83016521fc430e7dd81c9aa959db1c27c8a4



diff --git a/build.gradle b/build.gradle
index 8eb9643..c6b7c03 100644
--- a/build.gradle
+++ b/build.gradle
@@ -75,6 +75,7 @@
 dependencies {
     compile 'net.sf.jopt-simple:jopt-simple:4.6'
     compile group: 'com.google.guava', name: 'guava', version: '19.0'
+    compile group: 'it.unimi.dsi', name: 'fastutil', version: '7.2.0'
     compile group: 'org.apache.commons', name: 'commons-compress', version: '1.12'
     compile group: 'org.ow2.asm', name: 'asm', version: '5.1'
     compile group: 'org.ow2.asm', name: 'asm-commons', version: '5.1'