Fix fileno causing compile error when #defined

Error:
expected unqualified-id before '(' token
 int fmt::BufferedFile::fileno() const {

This is an issue with Android NDK and mingw32.

Silence some performance warnings in Visual Studio

Older Visual Studio versions are not shipped with stdint.h

in custom stream buffer implementation:

* basic_formatbuf -> internal::FormatBuf
* remove unused Traits template argument