Adds instruction for using Build2's package manager

(proper naming suggested by `build2`'s author)

I created a `nlohmann-json` package for `build2` (https://build2.org) that have been published on the central public repository https://cppget.org : https://cppget.org/nlohmann-json (at the moment you can see that there is only one version: 3.7.3).

The repository of the package code is there: https://github.com/build2-packaging/nlohmann-json and is owned by the build2 community now. (with build2 you can use either a repository like cppget.org or a git repository so both are useful to users for different use cases).

This patch just add the basic information as to how to use this library with build2.

Update README.md

Fix warning: ignoring return value

docs: add central repository as conan source to readme

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