Update the helper files so that the library can be compiled with newer compilers

[shibatch]

Checklist

• I have read the contributing guidelines.
• I have considered portability of my change across platforms and architectures.
• I have self-reviewed my code.
• I have commented my code where necessary.
• I have updated the documentation accordingly.
• I have added tests that prove my fix is effective or that my feature works.

What is the purpose of this pull request?

• This update is required to build the library with newer compilers, especially MSVC.

What changes did you make?

• Notation like ((vdouble) { +0.0, -0.0, +0.0, -0.0 }) is not compliant to the standards, and had to be replaced with vector-set intrinsics.
• Added explicit typecasting so that the code can be compiled with C++ which has strict type checking.

Does this PR relate to any existing issue?

This is part of the patches that implements #574.

This is manually rebased #586.

[blapie]

Looks good to me!

Not required for this patch but to claim support it would be good if we could build (and test) with MSVC in GitHub Actions. Created #588 for that.

Btw: I think you could have rebased without creating a new PR (sync fork with upstream, then rebase upstream/master onto existing branch, then force push branch to your fork).

[shibatch]

I know I can rebase without creating a new PR, but I am unfamiliar with git and wasn't confident I could do it right. I did this because if I rewrite the main branch by mistake, it could be quite messy.