RFR: 8357053: ZGC: Improved utility for ZPageAge

[Joel Sikström]

Hello,

This RFE improves utility for converting to/from, iterating over and defining structures that are indexed using the `ZPageAge` type.

Converting to/from ZPageAge and its underlying type (uint8_t, often just uint) is currently done via using static_cast. This works fine because sane values are converted in all use cases. However, to make conversion safer (and also more readable), I propose we add a `to_zpageage` and a corresponding `untype` that checks that the conversion is valid. Such conversion methods should be used instead of calling `static_cast<uint/ZPageAge>`.

We currently define a value called `ZPageAgeMax`, which is defined as `static_cast<uint>(ZPageAge::old)`. The majority of places that use this value actualy use `ZPageAgeMax + 1`, which is equivalent to the number of ages. Instead, I propose we define and use a value that represents the number of possible ages, called `ZPageAgeCount`.

Lastly, to make iterating over ages more accessible, I propose we create an intreface of enum iterators of ZPageAge. This will also create a foundation for generating values that require a ZPageAge in the future. Since the end of the enum iterators are exclusive, I've opted to use the following value as end for the iterators:

constexpr ZPageAge ZPageAgeLastPlusOne = static_cast<ZPageAge>(ZPageAgeCount);


I see us using either this or a sentinel/dummy value at the end of the enum class, but I prefer having a value similar to `ZPageAgeLastPlusOne` over a dummy value.

Testing:
* Currently running through Oracle's tier 1-4
* GHA

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Commit messages:
 - 8357053: ZGC: Improved utility for ZPageAge

Changes: https://git.openjdk.org/jdk/pull/25251/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25251&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357053
  Stats: 131 lines in 13 files changed: 81 ins; 7 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/25251.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25251/head:pull/25251

PR: https://git.openjdk.org/jdk/pull/25251