3.1.0 update

New this release

• Implementation of P1976R2 for gsl::span introduces explicit construction of fixed-length spans from dynamic spans.
  • See span_compatibility_tests.cpp for additional details.
  • PR: #887
• Better template argument deduction for gsl::span.
  • See span_tests.cpp for examples.
  • PR: #891
• Improved natvis for span and span derived types.
  • PR: #857
• Added CMake find_package version support.
  • PR: #879