Merge all cpp sources

[TurkeyMan]

Merged cpp source into one file as per @WalterBright's recommendations for correct usage of extern(C++).

[dlang-bot]

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Testing this PR locally

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[dnadlinger]

(Sorry, on mobile…)

[JinShil]

See https://forum.dlang.org/post/mailman.2861.1532969129.29801.digitalmars-d@puremagic.com for context ;-)

[Geod24]

Uh, that is obviously going to scale very well /s
Thanks for the context @JinShil

[jmdavis]

sigh I don't see how stuffing all of C++'s std in a single file is going to be anything other than an unmaintainable mess. There isn't a lot of code here right now, but if we ever end up with much of the STL and whatnot in druntime, then this file will become so huge that it would be insane. If the "correct" usage of extern(C++) involves shoving entire namespaces in a single file, then we're doing something very wrong.

[TurkeyMan]

sigh I don't see how stuffing all of C++'s std in a single file is going to be anything other than an unmaintainable mess. [...]

I know, right...