[SPARK-4795][Core] Redesign the "primitive type => Writable" implicit APIs to...

[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