Add -Wtype-limits feature

[jwnrt]

This flag catches easy to miss cases like this:

for (uint8_t i = 5; i >= 0; --i) {
  printf("hello!\n");
}

where a uint8_t will always be >= 0:

[jw type-limits]$ clang -Wtype-limits main.c    
main.c:5:25: warning: result of comparison of unsigned expression >= 0 is always true [-Wtautological-unsigned-zero-compare]
  for (uint8_t i = 5; i >= 0; --i) {
                      ~ ^  ~

GCC includes this in -Wextra, but surprisingly Clang doesn't even include it in -Wpedantic at the moment and you have to go all the way up to -Weverything.

I haven't tested this because I can't get the CRT repo to build at the moment, but maybe it's simple enough.

[jwnrt]

Removed the flag from the defaults