[SUGGESTION] Refactorings for to_cpp1.h file

[dutkalex]

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This PR contains a handful of small helper extractions to simplify the to_cpp1.h file, which reduces its length by about 1000 lines. Indeed, this file is very long and I believe that there are a lot of low hanging fruits like these ones... What do you think about that?

[dutkalex]

Behavior should be exactly the same, to the notable exception of the value of source_loaded in the case of a wrong file extension. Indeed, with the current implementation source_loaded == true in that case although file loading is bypassed, which I believe is a bug...

[hsutter]

Thanks for your pull request! It looks like this may be your first contribution to cppfront. I need to send you the Contributor License Agreement (CLA) but I don't have your email address -- please email me at first dot last at gmail. Once it's signed I can look at your pull request. Thanks again for your contribution.

[dutkalex]

@hsutter email sent :)

[dutkalex]

It seems like something went wrong here, but I won't try to fix it without knowing first if this is something you are interested in...

[dutkalex]

Seems like the problem has been fixed...