RFR: 8285743: Ensure each IntegerPolynomial object is only created once [v2]

[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.

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

  move singleton back into impl and make constructor private

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8476/files
  - new: https://git.openjdk.java.net/jdk/pull/8476/files/b24ca783..331e9519

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8476&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8476&range=00-01

  Stats: 54 lines in 8 files changed: 7 ins; 19 del; 28 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