Fix typo in `cheatsheet.md`

Fix  crash with out of bounds index (issue #572)

Properly create a CMake a library called chaiscript

This allows to properly use Chaiscript as submodule with CMake.

Add include directory to chai target.

This way, the path to the Chai's include directory is automagicaly
knowns by any CMake project using chai.

Update readme.txt and fix json namespace clash with nlohmann/json

Fixes #486, #506

Merge pull request #441 from ChaiScript/develop

change .clang-format and reformat code with clang-format 11

Revert "Add pair_conversion registration helper with unit test"

Add pair_conversion registration helper with unit test

I initially tried to use the existing .clang-format file,
but it does not match the code style (at least with clang-format 11)
and the formatting is not consistent across files.
Therefore, I decided to rewrite the .clang-format with some personal
preferences.

Used command
  find . -iname "*.hpp" -o -iname "*.cpp" | xargs clang-format -i -style=file

Fixes for #527, #537, #553, C++20, and various compiler warnings/errors reported by g++ 11.1, clang 11, and vs2019

Both are already set by cmake

The already used functionallity already
requires a more recent cmake version.
Short summary why a never version is required:
  CMAKE_CXX_STANDARD requires >=3.1
  CMAKE_COMPILER_IS_GNUCC requires >3.7
  ENABLE_LTO can be simplified with >3.11 (3.13 VS)