rustfst-v0.12.0

What's Changed

• Remove macos-10.15 and ubuntu-18.04 that have been deprecated by @Garvys in #221
• Bump rust to 1.54.0 by @Garvys in #222
• Sigma transitions by @Garvys in #223

Full Changelog: rustfst-v0.11.6...rustfst-v0.12.0