Fixes to the formatting facilities:
- Streaming StringRef objects works but there's something wrong.
  See the FIXME in common.h
- Added support for format specifiers %x %X and %o in format()
- Removed support for the compile time check of format literals.
  It's a nice trick but it's not worth the pain of writing non-standard
  code, implementing and testing a fallback trick for other compilers etc...