Remove unused FFI functions from yices2 backend.

In preparation for dynamically loading the yices2 library.
Check for and load the yices2 library at runtime.

Instead of at installation time. This should make it much easier to install
and use the yices2 library and avoid the need for a separate smten-yices2
package.
Moved yices2 backend into smten-lib package.

Now that it loads dynamically, there are no problems with it being there.
Hurray! :)