0.8.1

This bug-fix release of span lite contains the following changes.

Fixes:

• Fix span iterator deduction guide (#61, thanks @Ryan-rsm-McKenzie)

Changes:

• Add more tests for construction via deduction guides
• Add Visual Studio 2019 to the Appveyor build matrix
• Remove Visual Studio 9 2008 from AppVeyor CI
• Deactivate allow_failures on Travis