RFR: 8292758: put support for UNSIGNED5 format into its own header file [v2]

[John R Rose]

On Fri, 2 Sep 2022 00:04:17 GMT, John R Rose <jrose at openjdk.org> wrote:

>> Refactor code from inside of CompressedStream into its own unit.
>> 
>> This code is likely to be used in future refactorings, such as JDK-8292818 (replace 96-bit representation for field metadata with variable-sized streams).
>> 
>> Add gtests.
>
> John R Rose has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Merge branch 'master' of https://git.openjdk.org/jdk into compressed-stream
>  - 8292758: put support for UNSIGNED5 format into its own header file

The new header file presents the encoding algorithm by means of templates.
The template arguments in general are:
 - `ARY` - a logical base address for reads and writes of bytes
 - `OFF` - an integral type (of any size or signed-ness) providing an offset to `ARY`
 - `GET` and `SET` - function-like arguments (e.g., lambdas) which get or set bytes from an address logically of the form `a[i]` shaped like `ARY[OFF]`
 - `GFN` a lambda used when the application requires on-the fly resizing of an output buffer (of type `ARY`)

Defaults are set in such a way that any C++ types that natively support `a[i]` can be fully inferred, including the get/set behaviors.

In addition, there are small "gadgets" for reading a series of ints from a buffer, writing a series to a buffer, and sizing a series (which is faster than writing or reading).  These are not yet used.  However, prototyping of further use cases for this compression (particularly, `FieldInfo`) makes it clear that these are repeated tasks that "canned" templates will help with.

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PR: https://git.openjdk.org/jdk/pull/10067