RFR: 8285743: Ensure each IntegerPolynomial object is only created once

[Weijun Wang]

All `IntegerPolynimial`s are singletons now. Also, hand-coded implementations for Ed25519 and Ed448 are removed. They were not used since `FieldGen` starts generating classes for them.

No new regression test. This is a clean-up.

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Commit messages:
 - the fix

Changes: https://git.openjdk.java.net/jdk/pull/8476/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8476&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285743
  Stats: 530 lines in 10 files changed: 28 ins; 465 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8476.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8476/head:pull/8476

PR: https://git.openjdk.java.net/jdk/pull/8476