Test Windows library changes

[MatthewLM]

In preparation for adding MuSig2 as required in #36, the build scripts have been updated. This has not been tested on Windows. The following needs to be tested on the 36-dlcs branch:

• coinlib/bin/build_windows.dart produces a DLL that passes all tests.
• coinlib/bin/build_windows_crosscompile.dart produces a DLL that passes all tests.
• coinlib/bin/build_wsl.dart produces a DLL that passes all tests.
• The coinlib_flutter/example app builds and runs on Windows.

[MatthewLM]

The completed MuSig2 implementation is available on the master branch now. Windows support can be tested on that. coinlib_flutter and the example app will need temporary path dependencies to the coinlib local path to be tested prior to release.