[SPARK-5424][MLLIB] make the new ALS impl take generic ID types

[mengxr]

This PR makes the ALS implementation take generic ID types, e.g., Long and String, and expose it as a developer API.

TODO:

• make sure that specialization works (validated in profiler)

@srowen You may like this change:) I hit a Scala compiler bug with specialization. It compiles now but users and items must have the same type. I'm going to check whether specialization really works.

[SparkQA]

Test build #26350 has started for PR 4281 at commit 135f741.

• This patch does not merge cleanly.

[SparkQA]

Test build #26352 has started for PR 4281 at commit 96072c3.

• This patch merges cleanly.

[SparkQA]

Test build #26352 has finished for PR 4281 at commit 96072c3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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[SparkQA]

Test build #26350 has finished for PR 4281 at commit 135f741.

• This patch passes all tests.
• This patch does not merge cleanly.
• This patch adds the following public classes (experimental):
  • case class Rating[@specialized(Int, Long) ID](user: ID, item: ID, rating: Float)

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[mengxr]

Compared object allocation and performance against master. Having generic types didn't downgrade performance. I've merged this into master.