Add support for 64-bit Windows builds of Bossa and bossac using Visual Studio 2019

[dc42]

I did this conversion originally to make it easier for me to debug why Bossa 1.9.1 Windows build would not write flash memory on a SAMV71Q20B chip. Having done the conversion, I found that these new 64-bit Windows builds can program the ATSAMV71 chip.

The main changes were:

• Added Visual Studio 2019 project and solution files
• Added additional include files in the src/msvc folder to replicate some Linux library functions used by Bossa
• Removed the use of variable-length arrays, which is a gcc extension to C++. I replaced them with use of std::vector to allocate the memory and free it automatically when it goes out of scope.
• Added explicit casts where needed to resolve type conversion warnings for the Microsoft C++ compiler
• Removed an alignment flag in a call to wxWidgets that caused an assertion failure within wxWidgets 3.1.5 in the debug build.