[SPARK-4795][Core] Redesign the "primitive type => Writable" implicit APIs to make them be activated automatically

Try to redesign the "primitive type => Writable" implicit APIs to make them be activated automatically and without breaking binary compatibility.

However, this PR will breaking the source compatibility if people use `xxxToXxxWritable` occasionally. See the unit test in `graphx`.

Author: zsxwing <zsxwing@gmail.com>

Closes #3642 from zsxwing/SPARK-4795 and squashes the following commits:

914b2d6 [zsxwing] Add implicit back to the Writables methods
0b9017f [zsxwing] Add some docs
a0e8509 [zsxwing] Merge branch 'master' into SPARK-4795
39343de [zsxwing] Fix the unit test
64853af [zsxwing] Reorganize the rest 'implicit' methods in SparkContext

1 files changed, 1 insertions, 1 deletions

diff --git a/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
index 265827b334..f2c38e79c4 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
@@ -40,7 +40,7 @@ class ShortestPathsSuite extends FunSuite with LocalSparkContext {
       val graph = Graph.fromEdgeTuples(edges, 1)
       val landmarks = Seq(1, 4).map(_.toLong)
       val results = ShortestPaths.run(graph, landmarks).vertices.collect.map {
-        case (v, spMap) => (v, spMap.mapValues(_.get))
+        case (v, spMap) => (v, spMap.mapValues(i => i))
       }
       assert(results.toSet === shortestPaths)
     }