[legacy example] add darwin version detect (#1081)

* add null_terminated_c_str_v for a null terminated string literal

* fix __builtin_addressof(::fast_io::char_literal_v<ch_type, 0>

* string_view uses null_terminated_c_str_v

* Fix warnings caused by macromodel

Why do you not compile code with warnings enabled? this is so painful

* change string.h to lf

* overhual the implementation of string.h to remove the self-referencing and make it trivially_relocatable

* fix string append for empty string

* Add fuzzing tests for string
Add basic_string_view, basic_cstring_view and basic_string in fast_io namespace

* fuzzing test for string.cc

* add potential zero test

* fix warnings in string fuzzing

* [vector] add resize and vecstring fuzzing test

* [Fix] vector resize

* [vector] add assign

* [string] update begin_ptr and end_ptr in assign

* [libcxx] support -DLIBCXX_ENABLE_WIDE_CHARACTERS=Off

when user builds LLVM libc++ with wide characters disabled, the library cannot compileany more. fix it

* [legacy example] add darwin version detect

---------

Co-authored-by: hearyldp <[email protected]>

examples/0007.legacy/construct_fstream_from_syscall.cc

@@ -73,6 +73,9 @@ FILE* or fstream apis
 			fast_io::mnp::hex0x(__MSVCRT_VERSION__),
 			"\n"
 #endif
+#elif defined(__DARWIN_C_LEVEL)
+			"Darwin ",
+			__DARWIN_C_LEVEL, "\n"
 #endif
 #if defined(_LIBCPP_VERSION)
 																		 "LLVM libc++ ",