RFR: 8246797: A convenient method to read OPTIONAL element [v2]

[Weijun Wang]

> This code change adds new methods to `DerInputStream` to easily and safely read optional fields in a ASN.1 DER-encoded value. It also adds several wrapping methods to `DerValue` to avoid unnecessary memory copying when calling by an internal method.
> 
> The new methods are applied to `OAEPParameters` to show out how they work. A new regression test is added to show that not only the new methods are simper but they are safer and more correct.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  best practice for reading DEFAULT, switch expression, etc

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5221/files
  - new: https://git.openjdk.java.net/jdk/pull/5221/files/d921bbbd..3e7b09e2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5221&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5221&range=00-01

  Stats: 30 lines in 1 file changed: 4 ins; 13 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5221.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5221/head:pull/5221

PR: https://git.openjdk.java.net/jdk/pull/5221