Includes libmbt in Go module source

[rnbguy]

Closes: #145

Description

Following cgo docs, libmbt should be part of go module source.

go get github.com/informalsystems/modelator/go/pkg/core would be successful when this PR is merge.

Until then, go get github.com/informalsystems/modelator/go/pkg/core@9935d79 should work as a fix.

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For contributor use:

• If applicable, unit tests are written and added to CI. Current CI takes care of it.
• Ran go fmt, cargo fmt and etc. (or had formatting run automatically on all files edited)
• Updated relevant documentation (docs/) and code comments.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Re-reviewed Files changed in the Github PR explorer.
• Added a changelog entry, using unclog.

[rnbguy]

This includes mbt library for x86_64 architecture only. It supports following targets

• linux-gnu
• darwin
• windows-gnu