Convert source files encoded ISO-8859-1 to UTF-8 (except mpir.net)

[musicinmybrain]

Non-ASCII source files are currently encoded in a mixture of UTF-8 and ISO-8859-1. This PR standardizes on UTF-8, except files in the mpir.net directory.

[musicinmybrain]

Not touched are files in the mpir.net directory, because I have inadequate Windows domain knowledge to be confident I am not breaking something, and no ability to test on that platform. The following files remain that are neither UTF-8 nor ASCII:

• mpir.net/mpir.net/app.rc is UTF-16. This is fine, I think, as it is a generated file and MSVC recognizes UTF-16 automatically. It does only contain ASCII characters, so it could perhaps be transcoded—the only real advantage being that git would not consider it a binary file.
• mpir.net/mpir.net/AssemblyInfo.cpp is ISO-8859-1. This is the one that scares me, as it has a non-ASCII © symbol in a string literal. I suspect the result may be dependent on the developer’s system locale, but I am not sure how MSVC would treat it in a Unicode source file. I am not even sure if the right thing is happening now. It may be that the best thing would be to ASCII-fy this to (c).
• mpir.net/mpir.net/HugeInt.h is Windows-1252. Non-ASCII characters are exclusively in comments, so transcoding to UTF-8 would probably be fine.

[wbhart]

Thanks very much for the contribution. As the tests pass, I will merge this in 24 hours if there are no objections.