Reduce the header declarations (#1342)

- Since we have style rules to include foo.hpp at the top of its
  corresponding foo.cpp, this takes any headers included by foo.hpp
  as being also guaranteed for foo.cpp.

- Use C-style <foo.h> instead of <cfoo>, since the latter only
  guarantees putting symbols in the `std` namespace, which we are
  not using for C functions (e.g. `printf` not `std::printf`).

- Remove now-unused `__PRETTY_FUNCTION__` reporting

include/extern/utf8decoder.hpp

@@ -3,6 +3,8 @@
 #ifndef EXTERN_UTF8DECODER_H
 #define EXTERN_UTF8DECODER_H
 
+#include <stdint.h>
+
 uint32_t decode(uint32_t *state, uint32_t *codep, uint8_t byte);
 
 #endif // EXTERN_UTF8DECODER_H