Prepare for 1.2.0 release

[lopopolo]

Release focaccia 1.2.0.

focaccia is published on crates.io.

This release improves code quality, testing, and packaging.

NOTE: focaccia's license has been updated to properly reflect its usage of Unicode Data Files. The SPDX expression for this crate is now MIT AND Unicode-DFS-2016. #149

Improvements

• Remove dead trait impls in mapping modules to slightly improve compile times. Remove dead trait impls in mapping modules #113
• Use unchecked arithmetic in lookup function. Use unchecked arithmetic in lookup function #115
• Use .. to destructure enums when we do not care about their contents. Use .. to destructure enums when we do not care about their contents #135

Testing improvements

• Fix exhaustive tests to start at NUL instead of '0'. Fix exhaustive tests to start at NUL instead of '0' #114
• Add more case folding vector tests to crate root and tests for pathological cases. Add more case folding vector tests to crate root and tests for pathological cases #116
• Fix typo in test name, rename to clarify what it tests. Fix typo in test name, rename to clarify what it tests #127
• Add test that impl Display for NoSuchCaseFoldingScheme is non-empty. Add test that impl Display for NoSuchCaseFoldingScheme is non-empty #146

Packaging improvements

• Use precise crate version in Cargo.toml snippet in README. Use precise crate version in Cargo.toml snippet in README #124
• Ensure crate abides by the Unicode Data Files and Software License. Ensure crate abides by the Unicode Data Files and Software License #149