Replace leading underscores

[awf]

Current convention in our C++ files is to begin member variables with an underscore, which is to the letter of the C++ standard, but frowned upon. https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier

Workitem: replace struct A { int _a; } with struct A { int m_a; }