Add std.typecons.Rebindable2 for internal use.

[FeepingCreature]

Rebindable2 is a simplified version of std.typecons.Rebindable that cleans up every special case: classes, arrays and structs now have the same struct Rebindable2.

Whichever type you instantiate Rebindable2 with, you always get the same type out by calling value.get on the resulting container.

Also use this type to simplify the parts of Phobos I previously used Rebindable for.

This PR is the announced follow-up to #8735

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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#8768"

[atilaneves]

We need a better name than Rebindable2.

[FeepingCreature]

The thing is that Rebindable is genuinely a really good name for this type. You bind a value to a name when declaring a variable; Rebindable lets you dissolve that association and bind a new value to the same name.

[dkorpel]

We need a better name than Rebindable2.

Since the symbol isn't public, there's no hurry to give it a final name.