Add support for Windows x64 platform

[fiedukow]

This adds support for Windows x64 to swift-sodium.

This is achieved by:

• Adding windows 1.0.19 release of https://github.com/jedisct1/libsodium to the repo - similarly to how .a files are present for mac. Note, only x64, only v143 and only static variant is added as that's the only tested combination. This is added under Clibsodium-win
• FIXING the above release by moving sodium/crypto_onetimeauth_poly1305.h includes outside of extern "C" block as it supposed to be, it seems that this mistake was present in only this one file, but it was preventing compilation in Cxx interop mode.
• Providing specialized definition of Clibsodium target based on the above.
• Providing SODIUM_STATIC define for the target, to ensure static linkage of the library.
• Add extra linker path to test target so it can work.

This was tested by:

• Running swift test on this repo on windows.
• Adding this as dependency to arc project and checking that it properly links and produces same hashes as on macOS.

[fiedukow]

Probably worth changing the name of the path to libsodium.

This would be out of what upstream uses.

[compnerd]

Probably worth changing the name of the path to libsodium.

This would be out of what upstream uses.

Not sure I understand how this would be different from upstream. The local path that we use has no relation to what upstream does right?

[compnerd]

Minor nit on the naming of the local path, but I don't think that is worth blocking this on.