gcc 10 is miscompiling libdeflate on x86_64 at -O3 due to a regression
in how packed structs are handled
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94994).
Work around this by just always using memcpy() for unaligned accesses.
It's unclear that the "packed struct" approach is worthwhile to maintain
anymore. Currently I'm only aware that it's useful with old gcc's on
arm32. Hopefully, compilers are good enough now that we can simply use
memcpy() everywhere.
Update https://github.com/ebiggers/libdeflate/issues/64
* Bring in common headers and program code from xpack project
* Move program code to programs/
* Move library code to lib/
* GNU89 and MSVC2010 compatibility
* Other changes
