Refactor C++

[sc1f]

• Removes unnecessary string copies in emscripten.cpp
• Use std::move when possible in constructors for Table, View, and t_view_config
• Clarify purpose of filter_op in the view config, and refactor base methods to accept "or" as a valid filter op
• clean up filter parsing in emscripten.cpp

[texodus]

Looks good, thanks for the PR!