Bundle libz3 within scalaz3.jar

The loading of the libz3 is now deterministic, no more LD_LIBRARY_PATH
needed. libscalaz3/libz3 are always expected to be extracted to
<tmp>/SCALAZ3_<checksum>.

Works on linux and mac, compiles on windows but errors when running.
Release ScalaZ3 2.1 with libz3 bundled
Fix equals/hashcode invariant
Let's avoid being lazy, with proper hashcode definition
Remove warnings related to -Wreturn-type
Fix all kinds of warnings
Merge branch 'master' of https://github.com/epfl-lara/ScalaZ3