* src/hb-cff-interp-dict-common.hh: Use ull for unsigned int64_t

The llu suffix does not work for older Visual Studio versions
(pre-2013), but ull works for all the compilers that we attempt to
support.

* test/api: Fix build on pre-C99 compilers

Ensure variables are declared at the top of the block.

* src/hb-dsalgs.hh: Add specialization for hb_is_signed<> for __int8

Pre-Visual Studio 2010 does not consider __int8 (which is typedef'ed to
int8_t) to be equivilant to signed char, so the compiler cannot find the
corresponding hb_is_signed<> specialization that is needed.

The interesting thing is unsigned __int8 is considered to be equivilant
to unsigned char, so as the other types (short, int, long) that we look
for here, so only the specialization for __int8 is added here.

This will fix builds on Visual Studio 2008 at least.