Move CI to GitHub Actions

A few highlights relative to our existing CI:

- Add a FreeBSD 12.2 and 13.0 runner so we have some FreeBSD CI.
- Windows builds use msbuild with the Visual-Studio-provided clang toolchain to test clang
 - The matrix builds describe the axes of the matrix, not all points.
 - The Arm builds now cross-compile with a native clang and run the tests with qemu, rather than running the compiler, linker, and ctest all with qemu.
 
This also includes a fix for one of the tests that was doing `static_cast<unsigned int>(1) << 36`, which is undefined behaviour and was sometimes causing qemu to hang.  There is now an assert to catch this in the future.

src/ds/bits.h

@@ -48,6 +48,7 @@ namespace snmalloc
     constexpr T one_at_bit(S shift)
     {
       static_assert(std::is_integral_v<T>, "Type must be integral");
+      SNMALLOC_ASSERT(sizeof(T) * 8 > static_cast<size_t>(shift));
       return (static_cast<T>(1)) << shift;
     }