Add internal multi-backend entropy system

[0xEAB]

This provides a entropy (CSPRNG) multi-backend system for internal use.

• Dynamically loads getrandom() from libc on Linux.
• Provides a semi-automatic fallback mechanism (for backwards compatibility in std.random).
  Unlike getrandom() backwards compatibility shim #10741, this works without compiler assistance.

This patch should be reviewed thoroughly. Thanks!

Obsoletes dlang/dmd#21158.
Replaces #10739 and #10741.

[dlang-bot]

Thanks for your pull request, @0xEAB!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10748"

[LightBender]

For now I am OK with this because it's V2. But we need to have a long conversation about how to handle platform level interfaces in V3 that doesn't involve shoving everything into scattered internal folders.

[thewilsonator]

std/internal/entropy/entropy.d(59:6)[warn]: Public declaration 'forceEntropySource' is undocumented.
std/internal/entropy/common.d(15:6)[warn]: Public declaration 'EntropySource' has no documented example.
std/internal/entropy/common.d(48:6)[warn]: Public declaration 'EntropyStatus' has no documented example.
std/internal/entropy/common.d(59:8)[warn]: Public declaration 'EntropyResult' has no documented example.
std/internal/entropy/common.d(61:19)[warn]: Public declaration 'status' is undocumented.
std/internal/entropy/common.d(62:19)[warn]: Public declaration 'source' is undocumented.
std/internal/entropy/common.d(128:22)[warn]: A public function needs to contain a `Returns` section.
std/internal/entropy/common.d(128:27)[warn]: Public declaration 'isOK' has no documented example.
std/internal/entropy/common.d(128:52)[warn]: Parameter value isn't documented in the `Params` section.
std/internal/entropy/common.d(134:22)[warn]: A public function needs to contain a `Returns` section.
std/internal/entropy/common.d(134:27)[warn]: Public declaration 'isUnavailable' has no documented example.
std/internal/entropy/common.d(134:61)[warn]: Parameter value isn't documented in the `Params` section.

[thewilsonator]

Public declaration 'forceEntropySource' is undocumented.
```
Use `///`

[0xEAB]

IMHO it doesn’t make much sense to document functionality in std.internal anyway.
I’m a bit surprised that I don’t get away with it unlike other modules in said package.
To be honest, I’m tempted to just add exclusions for the files in dscanner.ini.

[0xEAB]

At this point, I think I have documented things to an extent that goes far beyond what could be reasonably required for an internal module. I’ll look into D-Scanner warnings that are still left, but I get more and more tempted to add exclusions. (I’ve already added one for the has-public-examples check.)

[0xEAB]

For now I am OK with this because it's V2. But we need to have a long conversation about how to handle platform level interfaces in V3 that doesn't involve shoving everything into scattered internal folders.

Sure thing!

I’m only improvising here and I wasn’t really sure how to organize the code. I went with a multi-module approach for encapsulation reasons. Although I have to admit the value of having that is questionable at best.
If someone were to suggest something better, I’d not be averse to implementing that change.

[ibuclaw]

Each of these package files are so small, it's probably not worth having them split per platform. Just one module and version as appropriate is perfectly fine.

[0xEAB]

Adam approved this a few weeks ago: #10748 (review)

[0xEAB]

I’m not sure who defeated whom, but the style check finally passes.