v1.2.0 release

This release adds improvements and changes, including some from user feedback.

• update cmake build, add options
• update API including more C++20 for_each, for_each_n, dispatch
• update enum_bitset ctor, using std::initializer_list
• added starts_from_zero
• updated and extended unit tests, srcloc tests
• update documentation
• fixed std::ostream missing error