Commit pre-built dlls for go-cosmwasm

[ethanfrey]

We should at least support:

• linux
• osx

and windows if possible

Add the build instructions to the Makefile and commit the stable dlls to source.
Anyone importing a tagged release from a go program should not have to worry about rust setup - they just use the prebuilts.

Only people working on this repo (or trying to run from an unreleased master) should care.

[ethanfrey]

It seems linux -> windows cross-compile is decently supported.
linux -> osx not so.

I found this article that might help: https://wapl.es/rust/2019/02/17/rust-cross-compile-linux-to-macos.html

[ethanfrey]

Maybe I will have to make native compile on the other platforms... just run virtual machines

• https://github.com/kholia/OSX-KVM
• https://dennisnotes.com/note/20180614-ubuntu-18.04-qemu-setup/

[ethanfrey]

Maybe this issue on osx rustc dylibs is relevant.
rust-lang/rust#28640

In any case, I definitely need a mac system to test this on