Changelog for utfcpp: 4.1.1 -> 4.2.0

2d8e20b Release 4.2.0
44fdb26 Update README with project history link
5caca3f Fix exception type in API_REFERENCE.md
87ca49f next16 properly reports errors
c5585ef Reordered validity checks to preserve the earlier behavior.
4f46292 Fixed some warnings in benchmark.cpp
2f74d3b Removed unused is_overlong_sequence()
ef75b56 Rewrote validate_next that returns a code point.
85d5927 Fix bench/compare_commits.py
3145774 Revert the "minor optimization for ASCII range" as it actually hurts performance.
ca9e9bc Forgot to include the change for benchmark.cpp
ee0712e Fix the benchmarking script
9fb955a Python script to easily compare performance of two commits.
bf99533 A minor optimization for ASCII range.