Fix #580: Make ByteBufferPool less memory-hungry

Reduce the buffer size to 8M based on the fact that the largest .nir file
currently in the repo is java.lang.Character at ~2M:

    > find . -name "*.nir" | xargs stat -f "%z %N" | sort -n | tail -5
    546211 ./scalalib/target/scala-2.11/classes/scala/collection/immutable/StreamViewLike$$anon$13.nir
    580412 ./scalalib/target/scala-2.11/classes/scala/collection/mutable/IndexedSeqLike$$anon$1.nir
    612962 ./javalib/target/scala-2.11/classes/java/math/BigDecimal.nir
    699387 ./javalib/target/scala-2.11/classes/java/util/Arrays$.nir
    2301253 ./javalib/target/scala-2.11/classes/java/lang/Character$.nir

util/src/main/scala/scala/scalanative/io/ByteBufferPool.scala

@@ -7,7 +7,7 @@ final class ByteBufferPool {
   private var buffers: List[ByteBuffer] = Nil
 
   private def alloc(): ByteBuffer = {
-    ByteBuffer.allocateDirect(64 * 1024 * 1024)
+    ByteBuffer.allocateDirect(8 * 1024 * 1024)
   }
 
   def reclaim(buffer: ByteBuffer): Unit = synchronized {