Extend std C++ include header list (fix #692)

[fedapo]

The new list is taken from the CPP2_USE_MODULES case and sorted alphabetically. This is a bit repetitive and could have been avoided, but I chose not to mess with the preprocessor if-else logic. I can change it, should several voices prefer that.

[hsutter]

Thanks! To make it easier for me to track what's being added, I went ahead and sorted the list myself (I think? it's been a long Sunday, please also fix any ordering oversights). So if you could please update the PR then I can see what headers are being added in the diff.

[fedapo]

@hsutter Sure. Done.

[JohelEGP]

Now this comment isn't true anymore.

[hsutter]

Ah, now I see what you're doing... I think you're suggesting that even non-pure/modules mode should just include all standard headers always? I was slightly worried about possible compile time impact, but then I suppose we're already doing that in -pure mode (when modules aren't available, which is most of the time right now in practice).

[hsutter]

Thanks @fedapo !

The original reason to provide the entire standard library in -pure-cpp2 mode was that module is import is blazing fast -- it's faster to use C++23 import std; than to compile iostream Hello World. So it made sense to try the experiment of just making it the default.

The reason I provided a way to include all std headers was as a fallback path for compilers that didn't have real module import yet (most of them) or C++23 import std; (essentially all of them).

I've tried out the suggestion here, and I'm seeing that #include of the entire standard library makes compiling small files (including the regression tests) noticeably sluggish -- not a whole lot, but noticeable.

However, I see value in providing a way to include the entire standard library even in non-pure-cpp2 mode, so I'm going to try out providing it as command line options. The idea is to make it "zero-overhead," i.e., let users can pay for the additional compilation work only if they ask for it:

  -im[port-std]           import the entire C++ standard library via 'import std;'
  -in[clude-std]          #include the entire C++ standard library - ignored if -import-std is set
  -p[ure-cpp2]            Allow Cpp2 syntax only - also sets -import-std

So now a mixed-Cpp1-and-Cpp2 file can easily opt into the convenience of either importing (fast!) or including (wait a couple of extra seconds) the entire standard library. Note -pure-cpp2 retains its current behavior because it implies -import-std.

I'll push that commit instead of this PR, but it was directly inspired by this. Again, thanks @fedapo !

[fedapo]

@hsutter Thanks for the notice.
Fine with me to get the full set of std includes through an ad hoc command-line switch.