Refactor the code base to use `std::string_view` more

[Strilanc]

• Fix stim.CompiledDetectorSampler.sample_write not supporting pathlib.Path paths
• Fix "~Hyz" not being included in exported quirk URLs when the gate was only used in a decomposition
• Switch a lot of pybind code over to std::string_view to reduce copies
  • Methods taking const std::string & and const char * now take std::string_view, saving a copy
  • Most cases of casting to std::string now cast to std::string_view, saving a copy
• Replace nearly all const std::string & arguments with std::string_view arguments
• Switch many compiled-time-data maps from std::map<std::string, *> to std::map<std::string_view, *>
  • Arg parsing maps given to find_enum_argument
  • "Two qubit gate piece data" used in diagram drawing
  • SVG gate data and 3d gate data in diagram drawing
  • File format data
• Move GateTarget::from_target_str to gate_target.h by moving several templated methods like read_single_target into that header
• Switch stim::print_circuit from taking a string indent to a counted indent
• Fix stim::Circuit and stim::DetectorErrorModel having implicit conversions from strings
• Changed stim::Gate::name from const char * to std::string_view (and changed all the name hashing methods to work on string views)
• Add a _c_str annotation to null-terminated string variables in most cases
• Switch from .data() to .c_str() when needing a null terminated string in most cases
• Delete unused SubCommandHelp::flag_set()