Extend support for std::string_view (C++ and Python)

[ManifoldFR]

New features

• Replace const std::string & by std::string_view in several free functions and class ctor/member functions
• Python bindings now support passing std::string_view as argument to C++ and returning std::string_view to Python (hi @jorisv)
• Added a hash function for std::string which supports transparent/heterogeneous lookup (through Boost's unordered_map which has the right find(), erase(), etc overloads), notably using std::string_view

Resolves #363