Add UTF string library (#49)

* Add UTF string library

* Combine unit tests

* Fix code style

* Refactor until append8/append16

* Refactor until `replace_invalid`

* Refactor until `is_valid`

* Refactor until `find_invalid`

* Refactor until `starts_with_bom`

* Refactor until `increment`/`decrement`

* Refactor until "string_class_and_literals"

* Refactor until string conversion

* Refactor class `iterator`

* Port test "utf8_invalid"

* Enable unicode string tests in CI

* Degrade `std::from_range` constructor

* Refactor iterator

* Add `bounded_prev`/`bounded_next`

Author: saki7

CMakeLists.txt

@@ -8,9 +8,9 @@ endif()
 
 project(iris VERSION 0.0.1 LANGUAGES CXX)
 
-if(NOT DEFINED IRIS_ROOT)
-    set(IRIS_ROOT "${CMAKE_CURRENT_LIST_DIR}")
-endif()
+set(IRIS_ROOT "${CMAKE_CURRENT_LIST_DIR}")
+set_property(GLOBAL PROPERTY IRIS_ROOT "${IRIS_ROOT}")
+
 
 # -----------------------------------------------------------------
 # Global settings