Fix warning on win64
pack: `__object_header` always returns unsigned values
array: Wrap `array_alloc` as a single statement
checkout: Remove unused vector
iconv: Do not fake an API when iconv is not available
clar: Fix warnings in GCC/Linux
Merge pull request #1933 from libgit2/vmg/gcc-warnings

Warnings for Windows x64 (MSVC) and GCC on Linux