C++20 modernization of mathematical utility functions

The header mathfns.h containing some inline mathematical utility functions is modernized:

• All uses of std::enable_if are eliminated.
• The function floorlog2() is reimplemented, resolving a TODO comment.
• Duplicated, closely related code is moved to this header.
• Unused includes are cleaned up.
• A missing include of iostream header is added to pixelstreamer.cpp, since that file references std::cerr.