Import C++ Standard Template Library

You can now use the hardest fastest and most dangerous language there is
with Cosmopolitan. So far about 75% of LLVM libcxx has been added. A few
breaking changes needed to be made to help this go smoothly.

- Rename nothrow to dontthrow
- Rename nodiscard to dontdiscard
- Add some libm functions, e.g. lgamma, nan, etc.
- Change intmax_t from int128 to int64 like everything else
- Introduce %jjd formatting directive for int128_t
- Introduce strtoi128(), strtou128(), etc.
- Rename bsrmax() to bsr128()

Some of the templates that should be working currently are std::vector,
std::string, std::map, std::set, std::deque, etc.

libc/mem/unhexstr.c

@@ -21,7 +21,7 @@
 #include "libc/mem/mem.h"
 #include "libc/str/str.h"
 
-nodiscard void *unhexstr(const char *hexdigs) {
+dontdiscard void *unhexstr(const char *hexdigs) {
   assert(strlen(hexdigs) % 2 == 0);
   return unhexbuf(malloc(strlen(hexdigs) / 2), strlen(hexdigs) / 2, hexdigs);
 }