This commit adds many new sections to the existing "Developing and building
MicroPython" chapter to make it all about the internals of MicroPython.
This work was done as part of Google's Season of Docs 2020.
Add working example code to provide a starting point for users with files
that they can just copy, and include the modules in the coverage test to
verify the complete user C module build functionality. The cexample module
uses the code originally found in cmodules.rst, which has been updated to
reflect this and partially rewritten with more complete information.
Support C++ code in .cpp files by providing CXX counterparts of the
_USERMOD_ flags we have for C already. This merely enables the Makefile of
user C modules to use variables specific to C++ compilation, it is still up
to each port's main Makefile to also include these in the build.
