String Theory 3.9

• CMake: Add namespaced library string_theory::string_theory for better CMake target integration. The previous non-namespaced target is still available, but may be removed in string_theory 4.0.
• CMake: Fix attempted multiple definition of string_theory target when the package is found more than once.
• Annotate functions with [[lifetimebound]] attribute where applicable.
• Update minimum CMake version to 3.14.
• Update GTest to 1.15.2 (NOTE: This means building the string_theory tests requires C++14 now)
  • NOTE: GTest is now fetched with FetchContent rather than being included as a submodule.