Fixes for -Wconversion

- casting into to char near tolower and constructing chars from ints
- unsigned int foo : 1 for bit flag
- raptor_stringbuffer_length() to unsigned int
Redisable -Wconversion - generated lexer too  noisy
Disable GCC -Wformat-literal in a few places using GCC pragma

Using #pragma GCC diagnostic <...>
from https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html
Replace type %d (for enums) with %u in error messages (-Wformat)
Disable "-Wformat-nonliteral" with GCC pragma for GRDDL error format