This commit adds code changes, compatibility functions, stubs and a build script
to build urbit binaries on MingW64. Some functionality is limited or missing:
terminal input and daemon mode is not available, graceful exit does not work,
and the binaries are not completely static and use (portable) MingW dlls.
To build the binaries, install the MSYS2 environment, check out or copy the urbit
repo and pill binaries, open a MingW64 shell and `cd pkg/urbit && ./build-mingw`.
This also removes nixcrpkgs and OSX cross compilation in favour of
compiling on the target. x86_64/musl targets are still supported
on Linux.
All sources are now managed via niv (see nix/sources.json) and Haskell
package sets are provided/organised via IOHK's haskell.nix.
Some effort has been made to expose similar top-level attributes for
development, but in some cases there have been changes. Please see
the comments in the top-level default.nix and ci.nix files for usage.
