README: Package in MSYS2 renamed to nlohmann-json

This is done to correctly match with the
corresponding package in Archlinux.

https://twitter.com/travisci/status/1197625080119021568

Update tests that generate CMake projects to use main project's C++ compiler

Fix msvc warnings

- Also fixes warning in msvc about possible loss of data in
  that size computation.

Update tests that generate CMake projects to use the CMAKE_CXX_COMPILER the main project was CMake'd with. Fixes #1747.

Fix quadratic destruction complexity

Make our intent more clear in destruction

Prevent stackoverflow caused by recursive deconstruction

Configure WhiteSource Bolt for GitHub

Coverity detected two "Memory - illegal accesses  (OVERRUN)" issues. Resizing the buffer should silence this warning.

closes #1824

Fix for #1647

Add restriction for tuple specialization of to_json